Add a "SymbolEngine" to support top-level ref resolution by symbol

Summary:
Ref T13490. I'm continuing to move toward modernizing "amend", which needs to load some objects by symbol at top level.

Add an API to support this.

Test Plan: Added an API caller to "arc inspect", ran it and hit the new code. Things seemed to work.

Maniphest Tasks: T13490

Differential Revision: https://secure.phabricator.com/D21094