Add an option for the toplevel derivation

nix/lib.nix

@@ -71,6 +71,10 @@ let
             modules = [
               extraArgsModule
               ./modules
+              {
+                _file = "${self.printAttrPos (builtins.unsafeGetAttrPos "a" { a = null; })}: inline module";
+                build = { inherit toplevel; };
+              }
             ] ++ modules;
           }).config;
 

nix/modules/default.nix

@@ -150,6 +150,11 @@
       };
 
       build = {
+        toplevel = lib.mkOption {
+          type = lib.types.pathInStore;
+          readOnly = true;
+        };
+
         scripts = lib.mkOption {
           type = lib.types.attrsOf lib.types.package;
         };