Add and populate a pathIndex column for OwnersPath

Summary: Ref T11015. This supports making path names arbitrarily long and putting a proper unique key on the table.

Test Plan:

• Migrated, checked database, saw nice digested indexes.
• Edited a package, saw new rows update with digested indexes.

Maniphest Tasks: T11015

Differential Revision: https://secure.phabricator.com/D19181