A repository need not contain any unique layers, if its images use only layers
mounted from other repositories. But, the catalog endpoint was looking for the
_layers directory to indicate that a directory was a repository.
Use the _manifests directory as the marker instead, since any repository with
revisions will contain a _manifests directory.
Signed-off-by: Adam Wolfe Gordon <awg@digitalocean.com>
a784441b62