Added test for, e.g.:

    import spack.pkg.builtin.mock.mpich
    import spack.pkg.builtin.mock.mpich as mpich
    from spack.pkg.builtin.mock.mpich import Mpich

Among others.  These ensure that direct package imports work so that
packages can be extended.

Package repositories now look like this:

    top-level-dir/
        repo.yaml
        packages/
            libelf/
                package.py
            mpich/
                package.py
            ...

This leaves room at the top level for additional metadata, source,
per-repo configs, indexes, etc., and it makes it easy to see that
something is a spack repo (just look for repo.yaml and packages).

Making distinction between install database and package repositories clearer.

- fix up routines in namespace trie.
- trie can now hold intermediate elements.
- trie now has a test case.

Conflicts:
	lib/spack/spack/__init__.py
	lib/spack/spack/directives.py
	lib/spack/spack/packages.py

- `remove_prefix` was modified to remove from the DB, but the package
  may not have been added to the DB yet when `remove_prefix` is called
  from `cleanup`.

- Made `remove_prefix` a pure utility function (it just removes the prefix)

- Added `installed_db.remove()` call only after the `remove_prefix` in
  `uninstall`.

Fixed typo : write_lock() -> write_transaction()

Features/testinstall cmd

dbus: disable systemd

clang : updated versions + depends_on(python)

ncurses: update package

paraview: add package

generalize one of the cases to handle many more kinds of formatting o…

architecture: use uname if available

GCC : added variant to packages for binutils, solve linking time error

- Code simplification ignored case where exception was raised.
- If LockError was raised, read and write counts were incremented erroneously.
- updated lock test.

them do have a source_path: use this instead to check whether the package
resources were successfully retrieved.

Added v5.2.2 for xz library

A first attempt; builds successfully. Some dependencies have hard-coded
dependencies where they shouldn't (e.g., hdf5 -> mpi, libxml -> python),
but that's a different rabbit hole.

Not necessary in spack. Also forcefully installs outside of the prefix.

llvm : added dependency on 'python@2.7:' for better portability

stage.fetch() was updated to raise a FetchError instead of calling tty.die().
Output is the same for spack install in case of a fetch error.