Provide a modern ("v3") API for querying files ("file.search")

Summary: Ref T11357. Implements a modern file.search for files, and freezes file.info.

Test Plan: Ran file.search from the Conduit console.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11357

Differential Revision: https://secure.phabricator.com/D17612