R1:8cb273a0532b

Add a unique key to OwnersPath on "<packageID, repositoryPHID, pathIndex>"

Summary:
Depends on D19181. Ref T11015. This nukes duplicates from the table if they exist, then adds a unique key.

(Duplicates should not exist and can not be added with any recent version of the web UI.)

Test Plan:
- Tried to add duplicates with web UI, didn't have any luck.
- Explicitly added duplicates with manual `INSERT`s.
- Viewed packages in web UI and saw duplicates.
- Ran migrations, got a clean purge and a nice…