Clean up support code for Home Manager as a submodule

This removes the `nixosSubmodule` option in favor of a new option
`submoduleSupport.enable`. This name better indicates that the
submodule mode applies to both NixOS and nix-darwin.

nix-darwin/default.nix

@@ -7,12 +7,10 @@ let
   cfg = config.home-manager;
 
   hmModule = types.submodule ({name, ...}: {
-    imports = import ../modules/modules.nix {
-      inherit lib pkgs;
-      nixosSubmodule = true;
-    };
+    imports = import ../modules/modules.nix { inherit lib pkgs; };
 
     config = {
+      submoduleSupport.enable = true;
       home.username = config.users.users.${name}.name;
       home.homeDirectory = config.users.users.${name}.home;
     };