Allow import/export of content not directly tied to repos

[daviddavis]

fixes #7252

Please be sure you have read our documentation on creating PRs:
https://docs.pulpproject.org/contributing/pull-request-walkthrough.html

[daviddavis]

Note this is WIP PR that only handles exporting the mapping so far. Here is an example of how this would be used:

class PackageResource(BaseContentResource):
    """
    Resource for import/export of rpm_package entities.
    """
    content_mapping = {}

    def _add_to_mapping(self, repo, uuids):
        if not uuids.exists():
            return

        self.content_mapping[repo.name] = list(map(str, uuids))

    def set_up_queryset(self):
        """
        :return: Packages specific to a specified repo-version.
        """
        packages = Package.objects.filter(pk__in=self.repo_version.content)
        self._add_to_mapping(self.repo_version.repository, packages.values_list("pulp_id",
                                                                                flat=True))

        tree = DistributionTree.objects.filter(pk__in=self.repo_version.content).first()
        if tree:
            for repo in tree.repositories():
                version = repo.latest_version()
                packages |= Package.objects.filter(pk__in=version.content)
                self._add_to_mapping(repo, packages.values_list("pulp_id", flat=True))

        return packages

    class Meta:
        model = Package
        import_id_fields = model.natural_key_fields()

And here is an example of the content_mapping that gets exported: https://gist.github.com/daviddavis/5d1575c6f89521bb3bff559afb7b5f2b.

[pulpbot]

Attached issue: https://pulp.plan.io/issues/7252

[goosemania]

Args are missing in the doc string

[daviddavis]

Since PEP 257 only applies to public methods, I only added a single line docstring to give a general overview of the method since I didn't feel like an entire docstring was necessary.

[goosemania]

Thanks!

[goosemania]

Travis is failing but I don't need to re-review