Implement an "Author's packages" Herald field for Differential

Summary: Ref T13480. Add an "Author's packages" field to Herald to support writing rules like "if affected packages include X, and author's packages do not include X, raise the alarm".

Test Plan: Wrote and executed rules with the field, saw a sensible field value in the transcript.

Maniphest Tasks: T13480

Differential Revision: https://secure.phabricator.com/D20947