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.
