Make all bin/* scripts locate their workflows dynamically

Summary:
Ref T2015. Not directly related to Drydock, but I bumped into this. All these scripts currently enumerate their workflows explicitly.

Instead, use PhutilSymbolLoader to automatically discover workflows. This reduces code duplication and errors (see all the bad extends this diff fixes) and lets third parties add new workflows (not clearly valuable?).

Test Plan: Ran bin/x help for each modified script.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2015

Differential Revision: https://secure.phabricator.com/D7840