Add modules

modules/default.nix

@@ -0,0 +1,39 @@
+# Licensed under GNU Lesser General Public License v3 or later, see COPYING.
+# Copyright (c) 2019 Alexander Sosedkin and other contributors, see AUTHORS.
+
+{ pkgs ? import <nixpkgs> { }, initialBuild ? false, config ? { } }:
+
+with pkgs.lib;
+
+let
+  homeDir = builtins.getEnv "HOME";
+  configFile = homeDir + "/.config/nixpkgs/nix-on-droid.nix";
+
+  hasConfigFile = builtins.pathExists configFile;
+
+  rawModule = evalModules {
+    modules = [
+      {
+        _module.args = { inherit pkgs; };
+      }
+    ]
+    ++ optional (!initialBuild && hasConfigFile) configFile
+    ++ optional (!initialBuild && !hasConfigFile && pkgs.config ? nix-on-droid) pkgs.config.nix-on-droid
+    ++ optional initialBuild config
+    ++ import ./module-list.nix;
+  };
+
+  failedAssertions = map (x: x.message) (filter (x: !x.assertion) rawModule.config.assertions);
+
+  module =
+    if failedAssertions != []
+    then throw "\nFailed assertions:\n${concatMapStringsSep "\n" (x: "- ${x}") failedAssertions}"
+    else showWarnings rawModule.config.warnings rawModule;
+in
+
+if initialBuild
+then module.config
+else {
+  inherit (module.config.build) activationPackage;
+  inherit (module.config.environment) path;
+}