- 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
- Add a PythonPackage class with build system support.
- Support build phases in PythonPackage
- Add a custom sanity check for PythonPackages
- Get rid of nolink dependencies in python packages
- Update spack create to use new PythonPackage class
- Port most of Python packages to new PythonPackage class
- Conducted a massive install and activate of Python packages.
- Fixed bugs introduced by install and activate.
- Update API docs on PythonPackage
* Removing the nobuild, nolink, and alldeps dependency types in favor of being explicit.
* This will help with maintenance going forward, as adding more dependency types won't affect existing declared dependencies in weird ways.
* default deptype is still `('build', 'link')`
