Binary caching bugfix: symlink relocation (#10073)

Binary caches of packages with absolute symlinks had broken symlinks.
As a stopgap measure, #9747 addressed this by replacing symlinks with
copies of files when creating binary cached packages.

This reverts #9747 and instead, either relative-izes the symlink or
rewrites the target. If the binary cache is created using '--rel' (as
in "spack buildcache create --rel...") then absolute symlinks will be
replaced with relative symlinks (in addition to making RPATHs relative
as before); otherwise they are rewritten (when the binary cache is
unpacked and installed).