Define common ID and PHID export fields in SearchEngine
Ref T13049. All exportable objects should always have these fields, so make them builtins.
This also sets things up for extensions (like custom fields).
Test Plan: Exported user data, got the same export as before.
Reviewed By: amckinley
Maniphest Tasks: T13049
Differential Revision: https://secure.phabricator.com/D18951