- 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
* When creating a tar of a package for a build cache, symlinks are
preserved (the corresponding path in the newly-created tarfile will
be a symlink rather than a copy of the file)
* Dont add external packages to a build cache
* When installing from binary cache, don't create install prefix until
verification is complete
Adds the "buildcache" command to spack. The buildcache command is
used to create gpg signatures for archives of installed spack
packages; the signatures and archives are placed together in a
directory that can be added to a spack mirror. A user can retrieve
the archives from a mirror and verify their integrity using the
buildcache command. It is often the case that the user's Spack
instance is located in a different path compared to the Spack
instance used to generate the package archive and signature, so
this includes logic to relocate the RPATHs generated by Spack.
