Add support for extraModules input

NixOS offers the possibility to pass a list of extraModules to
lib.nixosSystem which later get merged with the modules list and
baseModules.
This offers the possibility to read additional modules from environment
files or a custom module-list.nix.

NixOS implementation is
[here](https://github.com/NixOS/nixpkgs/blob/8601973cec591ab18357eb7c26c52769d990d1ed/nixos/lib/eval-config.nix#L33).

Dor nix-darwin an implementation was started in #592.
I restarted the implementation and removed the usage of
`NIXOS_EXTRA_MODULE_PATH` env variable because NixOS deprecated this
one.

eval-config.nix

@@ -17,6 +17,7 @@ in
 { lib
 , modules
 , baseModules ? import ./modules/module-list.nix
+, extraModules ? []
 , specialArgs ? { }
 , check ? true
 , enableNixpkgsReleaseCheck ? true
@@ -72,14 +73,14 @@ let
     _file = ./eval-config.nix;
     config = {
       _module.args = {
-        inherit baseModules modules;
+        inherit baseModules extraModules modules;
       };
     };
   };
 
-  eval = lib.evalModules (builtins.removeAttrs args [ "lib" "enableNixpkgsReleaseCheck" ] // {
+  eval = lib.evalModules (builtins.removeAttrs args [ "lib" "enableNixpkgsReleaseCheck" "extraModules" ] // {
     class = "darwin";
-    modules = modules ++ [ argsModule ] ++ baseModules;
+    modules = baseModules ++ extraModules ++ modules ++ [ argsModule ];
     specialArgs = { modulesPath = builtins.toString ./modules; } // specialArgs;
   });
 in

flake.nix

@@ -18,11 +18,13 @@
     lib = {
       evalConfig = import ./eval-config.nix;
 
-      darwinSystem = args@{ modules, ... }: self.lib.evalConfig (
+      darwinSystem = args@{ modules, extraModules ? [], ... }: self.lib.evalConfig (
         { inherit (nixpkgs) lib; }
         // nixpkgs.lib.optionalAttrs (args ? pkgs) { inherit (args.pkgs) lib; }
         // builtins.removeAttrs args [ "system" "pkgs" "inputs" ]
         // {
+          inherit extraModules;
+
           modules = modules
             ++ nixpkgs.lib.optional (args ? pkgs) ({ lib, ... }: {
               _module.args.pkgs = lib.mkForce args.pkgs;