Rename PhabricatorSearchEngine subclasses
Summary: Ref T5655. Rename PhabricatorSearchEngine subclasses for consistency.
Test Plan: arc unit
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: Korvin, epriestley
Maniphest Tasks: T5655
Differential Revision: https://secure.phabricator.com/D12669