The current implementation of `spack-python` will leave an extra shell
around while it runs.  That shell should really replace itself with
spack.

- [x] add exec to spack-python script

- remove the old LGPL license headers from all files in Spack
- add SPDX headers to all files
  - core and most packages are (Apache-2.0 OR MIT)
  - a very small number of remaining packages are LGPL-2.1-only

We moved to a new GitHub org! Now make the code and docs reflect that.

Added more test cases for multimethods.  In doing so, (re)discovered that
satisfies() really needs to handle dependencies properly.

Implemented support for dependencies in satisfies, but constrain() now
isn't consistent (as we do not currently constrain deps), so need to
implement that.

Virtual dependency support probably needs some deeper thought.  i.e.,
there is probably an intermediate DAG form that would make the needed
checks easier.  Right now we have to build ProviderIndexes to figure out
how virtual dependencies are set up.  If the vdep were preserved in the DAG,
then we could just check for things like incompatible providers directly.

New operations for manipulating spec and package DAGs.

For specs:
	flatten:   gather all deps to the root
	normalize: Merge constraints and make spec match package DAG

For packages:
	validate_dependencies: Make sure spec constraints in package DAG are sane.

Added tests for above methods.  Also added beginnings of concretization logic,
to turn abstract spec into a concrete one.

Still need proper tests for normalize().

- moved none_compare functions to util.none_high
  and util.none_low packages
- renamed utils.py to util package

Still organizing things.