Give "owners.search" a "paths" attachment and a default "owners" value

Summary:
Ref T9964.

• Add a "paths" attachment for fetching paths.
• Always load owners. We will need this to do policy checks in the future, anyway, and this data is not large, is very useful, and is reasonable to load unconditionally.

Test Plan:

• Queried packages via API.
• Edited packages (paths, owners).
• Created a package.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9964

Differential Revision: https://secure.phabricator.com/D14775