Policy - move some owners code into an editor class and check policy better

Summary: Ref T7094. We basically need to make sure folks can see repositories before making owners packages about code within. This cleans up things a little bit by moving a bunch of logic out of the storage class and into an editor class.

Test Plan: made a package and it worked! deleted a package and it worked! discovered buggy behavior in more complicated edits and filed T7127; note this bug exists before and after this diff.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7094

Differential Revision: https://secure.phabricator.com/D11652