feat(flake-module): add module path special args

That is, provide `modulesPath` and `extraModulesPath` in the arguments
to devshell configurations defined using the flake-parts module.  This
harmonizes the behavior of `devshell.mkShell` and the flake module.

Author: tomeon

flake-module.nix

@@ -21,7 +21,7 @@ in
           '';
 
           type = types.lazyAttrsOf (
-            types.submoduleWith { modules = import ./modules/modules.nix { inherit pkgs lib; }; }
+            types.submoduleWith (import ./modules/eval-args.nix { inherit pkgs lib; })
           );
           default = { };
         };

modules/default.nix

@@ -6,18 +6,13 @@ nixpkgs:
   extraSpecialArgs ? { },
 }:
 let
-  devenvModules = import ./modules.nix {
-    pkgs = nixpkgs;
-    inherit lib;
-  };
-
-  module = lib.evalModules {
-    modules = [ configuration ] ++ devenvModules;
-    specialArgs = {
-      modulesPath = builtins.toString ./.;
-      extraModulesPath = builtins.toString ../extra;
-    } // extraSpecialArgs;
-  };
+  module = lib.evalModules (
+    import ./eval-args.nix {
+      inherit lib extraSpecialArgs;
+      pkgs = nixpkgs;
+      modules = [ configuration ];
+    }
+  );
 in
 {
   inherit (module) config options;

modules/eval-args.nix

@@ -0,0 +1,17 @@
+# Arguments for `lib.evalModules` or `types.submoduleWith`.
+{
+  pkgs,
+  lib,
+  modules ? [ ],
+  extraSpecialArgs ? { },
+}:
+let
+  devenvModules = import ./modules.nix { inherit lib pkgs; };
+in
+{
+  modules = (lib.toList modules) ++ devenvModules;
+  specialArgs = {
+    modulesPath = builtins.toString ./.;
+    extraModulesPath = builtins.toString ../extra;
+  } // extraSpecialArgs;
+}