Crudely bridge legacy workflows into "arc help"

Summary:
Ref T13490. One of the biggest issues users are hitting in modern "arc" is that workflows don't appear in "arc help" until they're updated.

Since there's still some work to do and gluing them in isn't terribly difficult, at least get things connected for now.

Test Plan: Ran "arc help", "arc help diff".

Maniphest Tasks: T13490

Differential Revision: https://secure.phabricator.com/D21101