R1:81d051dbfa89

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