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