Switch Find Refs to use Hooks

Summary: Cleaned up Find References to use Typing_hooks, instead of global/references. Motivation is I want to reuse those hooks.

There's still some stuff to clean up (in another diff) to do with the 'identify symbol at position', before find_refs.ml can be deleted.

Reviewed By: @jwatzman

Differential Revision: D1744206

Signature: t1:1744206:1418781424:b32979558ad4245b187cd77d252167d7feacd7da