init: invokeai-dream2nix

Testing the grounds for the upcoming dream2nix v1.

Advantages:
  - get rid of ./packages
  - reduced amount of overlays/overriding
  - potentially simplifies upgrading invokeai (less dependency hell)
  - comes with some cool dream2nix features like `nix build .#invokeai-d2n.docs`

Shortcomings:
  - no amd/nvidia support yet.
  - dream2nix is unstable
  - the fetchPip fetcher is slow and ends up downloading more stuff than it needs to (not be a problem once cached)
  - interface for maintainers is different from nixpkgs