Split into per-host subflakes

[matrss]

With subflakes it should be possible to make changes affect only the system they are made on. Currently, if something in e.g. /hosts/hazuno is changed, the other hosts configuration hashes change as well, forcing an unnecessary update.

[matrss]

The cause for one host to affect another was adding this flake to every systems flake registry (and therefore copying the full nix source code into each output). By removing this entry from the registry in b0bdb80 the issue should be solved, even without splitting this into sub-flakes.

Splitting might still have some benefit in regard to being able to advance each systems nixpkgs input independent from each other, without having a bunch of nixpkgs inputs in the top-level flake.