Don't send mail to "uninteresting" auditors

Summary:
Fixes T11017. We add packages as "uninteresting" auditors so that we can query commits by package later.

Until recently, this didn't matter because we didn't send mail to packages. But now we do, so stop mailing them when they don't actually need to do anything.

Test Plan:

• Made a commit to a file which was part of a package but which I owned (so it does not trigger auditing).
• var_dump()'d mail "To:" PHIDs.
  • Before patch: included package.
  • After patch: no package.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11017

Differential Revision: https://secure.phabricator.com/D15970