Summary:
Ref T4222. Moves us toward a more modern Celerity CLI, and moves map discovery into the classtree. This is a little bit bulky (and means you can't ship completely standalone celerity maps) but has the advantage of being completely standard, and we could subclass maps into an auto-discovering map later if we have a need for it.

This doesn't affect the existing Celerity stuff. I'm going to build the new stuff in parallel, and then swap us over at the end.

Test Plan: Ran `bin/celerity map`, got reasonable-looking output.

Reviewers: btrahan, hach-que

Reviewed By: hach-que

CC: aran

Maniphest Tasks: T4222

Differential Revision: https://secure.phabricator.com/D7864