Filters for exploring Content relationships

[sdherr]

pulp_deb allows users to configure apt releases/distros, components, and what packages go in them by allowing users to add Releases, ReleaseComponents, PackageReleaseComponents, and Packages, all of which are types of generic pulpcore Content and all of which get mapped to a particular RepositoryVersion. That's fine, except pulp_deb is missing all filters that allow you to explore more than one link in that relationship tree.

So for example if I wanted to answer the question "What Packages are currently in this Release", I'd have to:

1. List ReleaseComponents in the current RepositoryVersion.
2. List PackageReleaseComponents in the current RepositoryVersion.
3. List Packages in the current RepositoryVersion.
4. Create a mapping on the client side that loops through Release -> ReleaseComponent -> PackageReleaseComponent -> Package to construct the answer to the question.

We can and should create some filters in pulp_deb that allow you to simply ask the question and get the answer. Converting this to a single db query would be a much more efficient and user-friendly way to solve the problem. I have a code change to supply these filters that is currently being tested, Pull Request incoming shortly.

[sdherr]

PR filed as #647