- 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
* Push default flag handlers into module scope
* Preserve backwards compatibility of builtin flag handler names
Ensure Spack continues to work for packages using the `Package.env_flags` idiom and equivalent.
* update docs and tests to match
* Update packages to match new syntax
