Simplify members() datasource function
Summary: Ref T4100. Implement members() in terms of the real projects datasource.
Test Plan: Used members() functions in Differential, browsed members() functions.
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T4100
Differential Revision: https://secure.phabricator.com/D12459