Discover workflows automatically.

Summary: Most scripts detect the relevant workflows automatically. Some scripts, however, use a hardcoded list of workflows.

Test Plan: Ran `./bin/aphlict`, `./bin/cache` and `./bin/phd`.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9564