Allow Herald to add package reviewers

Summary: Ref T10939. Packages are valid reviewers, so let Herald "Add Reviewers" and "Add Blocking Reviewers" actions add them.

Test Plan:

• Wrote a rule to add package reviewers.
• Hit the rule, saw a package reviewer added, viewed transcript.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10939

Differential Revision: https://secure.phabricator.com/D15917