Deprecate some contributions.

This is just a starting point to start discussion. I think we have a
few options with contributions:

- Stop accepting them altogether and move the ones we need/use/support
  into core commands
- Accept them on an unsupported basis and require they be added
  manually to the PATH
- Actively support and fix bugs in them all

Thoughts?
fwiw I've ported this to ruby while working on the deps rewrite.