Pulp

This thread [0] contains the discussion of why we should un-nest the urls.

This task is to
1. make each url endpoint a top level resource
2. Adjust the fields (on serializers) that reference resources that have moved
3. Remove drf-nested-routers and all code that uses it.
4. Delete the extra code for routing (urls.py)
(3 and 4 moved to https://pulp.plan.io/issues/3168)

Example:
v3/repositories/ - This endpoint is already top level, but it contains some extra code (which should be removed) that allows it to be a parent endpoint. It also has fields `importer` and `publishers` that reference resources that will move.

This should be done all at once. If it isn't, a fair amount of extra work will have to go into fixing fields that will be changed again. (Also, some of that will not be possible to get working in the intermediate due to a drf-nested bug.)

When this is finished, the top level API should look like this:

v3/repositories/
v3/importers/<type>/
v3/publishers/<type>/
v3/publications/
v3/distributions/
v3/artifacts/
v3/content/<type>/
v3/repositorycontents/
v3/tasks/
v3/users/
v3/workers/
v3/status/
v3/jwt/

[0]: https://www.redhat.com/archives/pulp-dev/2017-November/msg00090.html