Add Owners Package support for "Commit Hook: Content" Herald rules

Summary:
See PHI370. Support the "Affected packages" and "Affected package owners" Herald fields in pre-commit hooks.

I believe there's no technical reason these fields aren't supported and this was just overlooked.

Test Plan: Wrote a rule which makes use of the new fields, pushed commits through it. Checked transcripts and saw sensible-looking values.

Differential Revision: https://secure.phabricator.com/D19104