Support the debug .ddeb package type

Author: lieter (lieter)

Redmine Issue: 6757, https://pulp.plan.io/issues/6757

From Ubuntu Bionic onward, Canonical has patched debuild to change the suffix for debug ($package-dbgsym_$version) package from .deb to .ddeb.

When attempting to upload packages with this suffix, the tasks has the following error:

  File "/home/lieter/.virtualenvs/pulp3-devel/lib/python3.8/site-packages/rq/worker.py", line 886, in perform_job
    rv = job.perform()
  File "/home/lieter/.virtualenvs/pulp3-devel/lib/python3.8/site-packages/rq/job.py", line 664, in perform
    self._result = self._execute()
  File "/home/lieter/.virtualenvs/pulp3-devel/lib/python3.8/site-packages/rq/job.py", line 670, in _execute
    return self.func(*self.args, **self.kwargs)
  File "/home/lieter/src/Pulp/pulpcore/pulpcore/app/tasks/base.py", line 40, in general_create
    serializer.is_valid(raise_exception=True)
  File "/home/lieter/.virtualenvs/pulp3-devel/lib/python3.8/site-packages/rest_framework/serializers.py", line 243, in is_valid
    raise ValidationError(self.errors)

This is because the plugin (rightfully) assumes that all packages are called .deb (or .udeb for installer packages). I believe adding a 'DebugPackage` model that inherits from Package (changing only SUFFIX) would be a good addition to pulp_deb.

The workaround here would be to upload the .ddeb with a relatve_path ending with .deb, but I don't know if Ubuntu's apt/dpkg would like that (afaik, it accepts it).

Start with the validation failure shown at pulpcore/app/tasks/base.py:40, then trace the pulp_deb package model and its suffix handling. Confirm how .deb and .udeb packages are recognized, add coverage for .ddeb uploads, and verify that a .ddeb package is accepted without the relative-path workaround.