- 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
- spack.repository module is now spack.repo
- `spack.repo` is now `spack.repo.path()` and loaded lazily
- Added `spack.repo.get()` and `spack.repo.all_package_names()` as
convenience functions to simplify the new lazy interface.
- updated tests and code
* Add documentation for repositories and namespaces.
* Update and extend repository documentation per review.
- Also add `-N` argument for `spack spec`
