Repositories with no unique layers don't show up in _catalog #3172
Description
A repository can contain manifests, tags, etc. without having any unique layers, if all the images it contains use layers shared with some other repository. For example, if I do the following:
docker tag alpine:3 registry.example.com/other-repo:latest
docker push registry.example.com/other-repo:latest
docker tag alpine:3 registry.example.com/alpine:3
docker push registry.example.com/alpine:3
then the storage for registry.example.com will have the following contents:
docker/registry/v2/blobs/sha256/c9/c9b1b535fdd91a9855fb7f82348177e5f019329a58c53c47272962dd60f71fc9/data
docker/registry/v2/blobs/sha256/dd/ddba4d27a7ffc3f86dd6c2f92041af252a1f23a8e742c90e6e1297bfa1bc0c45/data
docker/registry/v2/blobs/sha256/e7/e7d92cdc71feacf90708cb59182d0df1b911f8ae022d29e8e95d75ca6a99776a/data
docker/registry/v2/repositories/alpine/_manifests/revisions/sha256/ddba4d27a7ffc3f86dd6c2f92041af252a1f23a8e742c90e6e1297bfa1bc0c45/link
docker/registry/v2/repositories/alpine/_manifests/tags/3/current/link
docker/registry/v2/repositories/alpine/_manifests/tags/3/index/sha256/ddba4d27a7ffc3f86dd6c2f92041af252a1f23a8e742c90e6e1297bfa1bc0c45/link
docker/registry/v2/repositories/other-repo/_layers/sha256/c9b1b535fdd91a9855fb7f82348177e5f019329a58c53c47272962dd60f71fc9/link
docker/registry/v2/repositories/other-repo/_layers/sha256/e7d92cdc71feacf90708cb59182d0df1b911f8ae022d29e8e95d75ca6a99776a/link
docker/registry/v2/repositories/other-repo/_manifests/revisions/sha256/ddba4d27a7ffc3f86dd6c2f92041af252a1f23a8e742c90e6e1297bfa1bc0c45/link
docker/registry/v2/repositories/other-repo/_manifests/tags/latest/current/link
docker/registry/v2/repositories/other-repo/_manifests/tags/latest/index/sha256/ddba4d27a7ffc3f86dd6c2f92041af252a1f23a8e742c90e6e1297bfa1bc0c45/link
Note that there's no _layers directory under alpine since all its layers are contained in other-repo.
Because of the way _catalog is implemented in distribution, this means alpine won't show up in the _catalog response.
It seems like an easy fix for this would be to use _manifests rather than _layers as the indicator that a directory is a repository. I've tried this and it appears to work fine. Would this cause any problems? If not, I'll submit a PR for it.