Consider packages when calculating Differential authority

Summary:
Ref T10939. This has no effect yet since packages can not actually become reviewers, I'm just inching toward support.

• When searching for "responsible users", include revisions that need review by packages you have authority over.
• When calculating review authority, include authority over packages you are a member of (these currently never exist).

Test Plan:
This isn't reachable so I just var_dump()'d stuff and looked at the generated queries, which appeared correct/reasonable.

I'll vet this more thoroughly once packages can actually become reviewers.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10939

Differential Revision: https://secure.phabricator.com/D15909