Draft: Plugins

This a somewhat large and fundamental change to the plugin backend system. It introduces the idea of swappable backends. The idea is that we can use a mock Drupal.org API backend while we develop the frontend, etc - and this also helps us define the API we need from D.O.

Also, while the real D.O. backend is being developed, work can continue in other areas by using the mock API.

Lastly, it allows enterprises to create their own endpoints / backends of supported modules, possibly enhanced with a library of their own custom modules which are available (and/or patched versions, etc).

CURRENT STATE: while the swappable backend system is in place, I have not yet decided on the API structure I want from the front-end to the Drupal site, so the frontend is currently broken. We need to hash that out, then decide how to mock the API.