R1:591d50008f90

Allow projects to own packages

Summary:
- The UI is pretty straightforward, since Handle just works (tm)
- Added two methods to the owners object to handle the new layer of
indirection. Then ran git grep PhabricatorOwnersOwner and changed
callsites as appropriate.

Sending this to get a round of feedback before I test the non-trivial
changes in this diff.

Test Plan:
- owners tool: edit, view, list for basic functionality.
- phlog for the two new methods I added

Reviewers: epriestley, blair…