Prefer llnl.util.symlink.readlink to os.readlink (#44126)

Symlinks on Windows can use longpath prefixes (\\?\); these are fine
in the context of win32 API interactions but break numerous facets of
Spack behavior that rely on string parsing/matching (archiving,
binary distributions, tarball extraction, view regen, etc).

Spack's internal readlink method (llnl.util.symlink.readlink)
gracefully handles this by removing the prefix and otherwise behaving
exactly as os.readlink does, so we should prefer that in all cases.