Make lookupFileInformation() a private API on CelerityResourceMap

Summary: Ref T4222. Currently, this exposes a bunch of information about the Celerity internals. This information is difficult to preserve exactly with the new maps. Strengthen the API by providing more specific capabilities.

Test Plan: Regenerated map, browsed around.

Reviewers: btrahan, hach-que

Reviewed By: hach-que

CC: aran

Maniphest Tasks: T4222

Differential Revision: https://secure.phabricator.com/D7867