Merge nixos-generators with nixpkgs

[zimbatm]

nixos-generators is a small wrapper around NixOS modules.

If we could get this upstreamed, it would be better maintained, as most of the bugs posted here are actually upstream issues.

This would also open the road to having a nixos-rebuild image --format <format> type CLI.

What needs to happen:

1. We need a unified interface that exposes all these different image types in one place. Like a system output of NixOS.
2. Bonus: make this part of nixos-rebuild.

[Mic92]

In nix we have a bundler interface. For consistency it could have the same name but on the other hand, "image" is way more common.

[zimbatm]

@phaer is working on this. See NixOS/nixpkgs#349624 and NixOS/nixpkgs#347275