Fix packages(project) to work properly and add it to "MailableFunctionDatasource"

Summary:
Ref T13210. See PHI937. This function datasource isn't quite implemented correctly: it doesn't resolve package(project) properly, since the logic only handles users.

This blames back to D14013, where it looks like packages(..) was added mostly as a general nice-to-have as part of a larger modernization change.

Test Plan: Ran a packages(project) query in Differential, got accurate results (previously: no results).

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13210

Differential Revision: https://secure.phabricator.com/D19747