- [x] add `concretize.lp`, `spack.yaml`, etc. to licensed files
- [x] update all licensed files to say 2013-2021 using
`spack license update-copyright-year`
- [x] appease mypy with some additions to package.py that needed
for oneapi.py
- spack.compilers.find_compilers now uses a multiprocess.pool.ThreadPool to execute
system commands for the detection of compiler versions.
- A few memoized functions have been introduced to avoid poking the filesystem multiple
times for the same results.
- Performance is much improved, and Spack no longer fork-bombs the system when doing a `compiler find`
- 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
