- in many files, regular strings were used in places where raw strings
should've been used.
- convert these to raw strings and get rid of new flake8 errors
- 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
