Before this PR, packages.yaml files that contained an
empty "paths" or "modules" attribute were not updated
correctly, since the update function was not reporting
them as changed after the update.

This PR fixes that issue and adds a unit test to
avoid regression.

This commit adds output to the "spack external find"
command to inform users of the result of the operation.

It also fixes a bug introduced in #17804 due to the fact
that a function was not updated to conform to the new
packages.yaml format (_get_predefined_externals).

* Handle uninstalled rootspecs in buildcache

- Do not parse specs / find matching specs when in an environment and no
package string is provided
- Error only when a spec.yaml or spec string are not installed. In an
environment it is fine when the root spec does not exist.
- When iterating through the matched specs, simply skip uninstalled
packages

fixes #18031

With this fix "spack config update" can update YAML
files that contain comments, while previously it
couldn't.

* Run Python2.6 unit tests on Github Actions
* Skip url tests on Python 2.6 to reduce waiting times
* Skip foreground background tests on Python 2.6 to reduce waiting times
* Removed references to Travis in the documentation
* Deleted install_patchelf.sh (can be installed from repo on CentOS 6)

* Buildcache:
   * Try mocking an install of quux, corge and garply using prebuilt binaries
   * Put patchelf install after ccache restore
   * Add script to install patchelf from source so it can be used on Ubuntu:Trusty which does not have a patchelf pat package. The script will skip building on macOS
   * Remove mirror at end of bindist test
   * Add patchelf to Ubuntu build env
   * Revert mock patchelf package to allow other tests to run.
   * Remove depends_on('patchelf', type='build') relying instead on
   * Test fixture to ensure patchelf is available.

* Call g++ command to build libraries directly during test build

* Flake8

* Install patchelf in before_install stage using apt unless on Trusty where a build is done.

* Add some symbolic links between packages

* Flake8

* Flake8:

* Update mock packages to write their own source files

* Create the stage because spec search does not create it any longer

* updates after change of list command arguments

* cleanup after merge

* flake8

- [x] Remove references to `master` branch
- [x] Document how release branches are structured
- [x] Document how to make a major release
- [x] Document how to make a point release
- [x] Document how to do work in our release projects

- [x] remove master from github actions
- [x] remove master from .travis.yml
- [x] make `develop` the default branch for `spack ci`

* Move flake8 tests on Github Actions

* Move shell test to Github Actions

* Moved documentation build to Github Action

* Don't run coverage on Python 2.6

Since we get connection errors consistently on Travis
when trying to upload coverage results for Python 2.6,
avoid computing coverage entirely to speed-up tests.

`spack -V` stopped working when we added the `releases/latest` tag to
track the most recent release. It started just reporting the version,
even on a `develop` checkout. We need to tell it to *only* search for
tags that start with `v`, so that it will ignore `releases/latest`.

`spack -V` also would print out unwanted git eror output on a shallow
clone.

- [x] add `--match 'v*'` to `git describe` arguments
- [x] route error output to `os.devnull`

* Change buildcache create `NoOverwriteException` back to a warning.

`spack buildcache list` was trying to construct an `Arch` object and
compare it to `arch_for_spec(<spec>)`. for each spec in the buildcache.
`Arch` objects are only intended to be constructed for the machine they
describe. The `ArchSpec` object (part of the `Spec`) is the descriptor
that lets us talk about architectures anywhere.

- [x] Modify `spack buildcache list` and `spack buildcache install` to
      filter with `Spec` matching instead of using `Arch`.

- [x] Make it easier to get a `Spec` with a proper `ArchSpec` from an
      `Arch` object via new `Arch.to_spec()` method.

- [x] Pull `spack.architecture.default_arch()` out of
      `spack.architecture.sys_type()` so we can get an `Arch` instead of
      a string.

* Loosen Axom's variants, add shared variant for axom, fix clang/xlf rpath'ing problem on blueos

* Fix flake8

* Add main branch to list of known git branches

The modifications in 193e8333
introduced a bug in the loading of compiler modules, since a
function that was expecting a list of string was just getting
a string.

This commit fixes the bug and adds an assertion to verify the
prerequisite of the function.

Packages can implement “detect_version” to support detection
of external instances of a package. This is generally easier
than implementing “determine_spec_details”. The API for
determine_version is similar: for example you can return
“None” to indicate that an executable is not an instance
of a package.

Users may implement a “determine_variants” method for a package.
When doing external detection, executables are grouped by version
and each group results in a single invocation of “determine_variants”
for the associated spec. The method returns a string specifying
the variants for the package. The method may additionally return
a dictionary representing extra attributes for the package.

These will be stored in the spec yaml and can be retrieved
from self.spec.extra_attributes

The Spack GCC package has been updated with an implementation
of “determine_variants” which adds the following extra
attributes to the package: c, cxx, fortran

The YAML config for paths and modules of external packages has
changed: the new format allows a single spec to load multiple
modules. Spack will automatically convert from the old format
when reading the configs (the updates do not add new essential
properties, so this change in Spack is backwards-compatible).

With this update, Spack cannot modify existing configs/environments
without updating them (e.g. “spack config add” will fail if the
configuration is in a format that predates this PR). The user is
prompted to do this explicitly and commands are provided. All
config scopes can be updated at once. Each environment must be
updated one at a time.

Fixes #17262

- This simplifies Spec.format somewhat
- Makes code to generate deptype strings (e.g., '[blrt]') reusable

* Fujitsu compiler: Accept alphabet as version.

* Fujitsu copiler: Updated test pattern.

`spack -V` stopped working when we added the `releases/latest` tag to
track the most recent release. It started just reporting the version,
even on a `develop` checkout. We need to tell it to *only* search for
tags that start with `v`, so that it will ignore `releases/latest`.

`spack -V` also would print out unwanted git eror output on a shallow
clone.

- [x] add `--match 'v*'` to `git describe` arguments
- [x] route error output to `os.devnull`

* For detecting Cray: CRAYPE_VERSION is not used, but MODULEPATH

* Fix typo and write Cray with a capital

* Change buildcache create `NoOverwriteException` back to a warning.

`spack buildcache list` was trying to construct an `Arch` object and
compare it to `arch_for_spec(<spec>)`. for each spec in the buildcache.
`Arch` objects are only intended to be constructed for the machine they
describe. The `ArchSpec` object (part of the `Spec`) is the descriptor
that lets us talk about architectures anywhere.

- [x] Modify `spack buildcache list` and `spack buildcache install` to
      filter with `Spec` matching instead of using `Arch`.

- [x] Make it easier to get a `Spec` with a proper `ArchSpec` from an
      `Arch` object via new `Arch.to_spec()` method.

- [x] Pull `spack.architecture.default_arch()` out of
      `spack.architecture.sys_type()` so we can get an `Arch` instead of
      a string.

* Run Python2.6 unit tests on Github Actions
* Skip url tests on Python 2.6 to reduce waiting times
* Skip foreground background tests on Python 2.6 to reduce waiting times
* Removed references to Travis in the documentation
* Deleted install_patchelf.sh (can be installed from repo on CentOS 6)

Relative paths in views have been broken since #17608 or earlier.

- [x] Fix by passing base path of the environment into the `ViewDescriptor`.
      Relative paths are calculated from this path.

Relative paths in views have been broken since #17608 or earlier.

- [x] Fix by passing base path of the environment into the `ViewDescriptor`.
      Relative paths are calculated from this path.

Relocation of sbang needs to be done when the spack prefix changes even if the install tree has not changed. (#17455)

Relocate rpaths for all binaries, then do text bin replacement if the rpaths still exist after running patchelf/otool (#17418)

A bug was introduced in #13100 where ChildErrors would be redundantly
printed when raised during a build. We should eventually revisit error
handling in builds and figure out what the right separation of
responsibilities is for distributed builds, but for now just skip
printing.

- [x] SpackErrors were designed to be printed by the forked process, not
      by the parent, so check if they've already been printed.
- [x] update tests