R1:e0dea4c4862f

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…