Rename the _phutil_date_format function for consistency

Summary: Ref T5655. Depends on D11188.

Test Plan: grep

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T5655

Differential Revision: https://secure.phabricator.com/D11189