Convert Files to SearchFields

Summary:
Ref T8441. Ref T7715.

• Update FileSearchEngine.
• Nothing too special/fancy.

Test Plan:

• Searched for various files.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7715, T8441

Differential Revision: https://secure.phabricator.com/D13175