- 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
This commit continues the changes discussed in #8823 by creating a new
"apple-libunwind" placeholder package that supplies instructions for
how to configure packages.yaml to register Apple's libunwind
implementation with spack. This package also provides the "unwind"
virtual package representing the libunwind base API. The clang-apple
compiler version should be specified in packages.yaml so that
apple-unwind is only used with Apple's clang compiler, not the stock
LLVM compiler.
