R1:ce78bf1de4e5

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…