Inject metadata when creating a filesystem symlink for a non-symlink artifact.

[tjgq]

A ctx.actions.symlink whose output is a declare_file or declare_directory
(as opposed to a declare_symlink) has "copy" semantics, i.e., the output
artifact is indistinguishable from the referent except for its name; the
symlink is just a filesystem-level optimization to avoid an actual copy,
and is transparently resolved when collecting the action output metadata.

When the symlink points to an artifact that was built remotely and without the
bytes, we currently must download it before executing the symlink action, so
that the output metadata can be constructed from the local filesystem. This
change short-circuits the filesystem traversal by injecting output metadata,
which is identical to the input plus a pointer to the original path. This is
used by the prefetcher to avoid downloading the same files multiple times when
they're symlinked more than once.

(An alternative would have been to teach all of the RemoteActionFileSystem
methods to resolve symlinks by patching together the local and remote metadata,
but that would have resulted in an awful lot of complexity.)

Fixes #15678.