feat(modules): load local icedos modules from extra-modules dir

[IceDBorn]

This PR adds support for loading local icedos modules, instead of needing an entire local repo for loading a local module.

Example icedos.nix:

{ ... }:

{
  inputs = {
    aagl = {
      url = "github:ezKEa/aagl-gtk-on-nix";
      inputs.nixpkgs.follows = "nixpkgs";
    };
  };

  outputs.nixosModules =
    { inputs, ... }:
    [
      inputs.aagl.nixosModules.default

      {
        nix.settings = inputs.aagl.nixConfig; # Set up Cachix
        programs.anime-game-launcher.enable = true; # Adds launcher and /etc/hosts rules
      }
    ];
}