Summary:
Ref T5655. It is superfluous to include "base" in the name of an abstract base class. Furthermore, it is not done consistently within the code base.
In order to retain compatibility with external code, I have kept the `ArcanistBaseWorkflow` class (which trivially extends from `ArcanistWorkflow`), but it is now deprecated and should output a warning message. Similarly for `ArcanistBaseUnitTestEngine`.
Test Plan: Created a workflow which extends from `ArcanistBaseWorkflow`. Executed the workflow and saw a deprecation warning.
Reviewers: #blessed_reviewers, epriestley
Reviewed By: #blessed_reviewers, epriestley
Subscribers: epriestley, Korvin, aurelijus
Maniphest Tasks: T5655
Differential Revision: https://secure.phabricator.com/D9983
