app1.nix

{ config, lib, pkgs, ... }:

with lib;

let
  cfg = config.app1;
in {
  options = {
    app1.foo = mkOption {
      description = "option foo";
      type = types.string;
    };
  };
  config = {
    system.build.app1 = pkgs.writeScriptBin "app1" ''
      #!/usr/bin/env bash
      export PATH=${config.system.build.app1_internal.env}/bin/
      hello
    '';
    system.build.app1_internal.env = pkgs.buildEnv {
      name = "app1-env";
      paths = with pkgs; [ hello ];
    };
  };
}

common.nix

{ lib, ... }:

with lib;

{
  options = {
    system.build = mkOption {
      internal = true;
      default = {};
      description = "Attribute set of derivations used to setup the system.";
    };
  };
}

config.nix

{ ... }:

{}

default.nix

let
  pkgs = import <nixpkgs> {};
  pkgsModule = rec {
    _file = ./default.nix;
    key = _file;
    config = {
      nixpkgs.system = pkgs.lib.mkDefault builtins.currentSystem;
    };
  };
  eval = pkgs.lib.evalModules {
    prefix = [];
    check = true;
    modules = [
      ./app1.nix ./config.nix ./common.nix
      <nixpkgs/nixos/modules/misc/nixpkgs.nix>
      pkgsModule
    ];
    args = {};
  };
in eval