By default the reachable set() includes all file digests from a
given reachable directory/tree digest, however this does not
guarantee the file objects are actually reachable.
testutils/artifactshare.py has_artifact() ascertained if a given
artifact is available by ensuring any 'reachable' digests are
actually on the filesystem, this is made redudant with the optional
check_exists objpath check during the reachable set creation.