- 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
- Generating the HTML from for >2300 packages from RST in Sphinx seems to
take forever.
- Add an option to `spack list` to generate straight HTML instead.
- This reduces the doc build time to about a minute (from 5 minutes on a mac laptop).