Fix visibility of various ArcanistWorkflow methods

Summary: Ref T6822.

Test Plan: Visual inspection. These methods are only called from within the ArcanistShellCompleteWorkflow class.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T6822

Differential Revision: https://secure.phabricator.com/D11238