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