Make edge types modular

Summary:
Ref T5245. I want to add a new capability to edge types, which is a good opportunity to move away from `PhabricatorEdgeConfig`, which isn't modular.

This is basically the same as the modularization of PHID types, which has worked well. Add `PhabricatorEdgeType` and provide an adaption layer for the existing code.

This has no runtime changes, except the fixed edge constant.

Test Plan: Ran `var_dump(PhabricatorEdgeType::getAllTypes())` and got reasonable looking output.

Reviewers: chad, btrahan, joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Maniphest Tasks: T5245

Differential Revision: https://secure.phabricator.com/D9837