RPM Support

Steps to reproduce¶

1. Sync a repo with a distribution tree such as https://fixtures.pulpproject.org/rpm-distribution-tree/
2. Delete the repo and run orphan cleanup
3. Go to an endpoint such as /pulp/api/v3/content/rpm/packages/ and observe that there's still content in the database

I did some debugging and it turns out the subrepos are still in the database and they're holding on to content.

This came up while trying to update our pulp_rpm tests to use the new rpm-distribution-tree fixture. Content sticks around which presents a problem for later tests that try to upload content and then hit a duplicate error. See the test failures here: https://github.com/pulp/pulp_rpm/pull/1832