[conduit: query owners packages by repository]

Summary:
There wasn't an easy way to see all the packages that cover a particular
directory. This adds the option

This is useful to see what's covered within a repo.

Test Plan:
* Queried with a valid repository and path - got the results
as before.
* Queried with a repository and no path - got an error as before.
* Queried with a repository and an asterix as the path - got all the
packages within the repository.

Revert Plan: sure

Other Notes:

Reviewers: epriestley, edward

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T3016

Differential Revision: https://secure.phabricator.com/D5763