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.

modules/misc/submodule-support.nix

@@ -0,0 +1,19 @@
+{ lib, ... }:
+
+with lib;
+
+{
+  meta.maintainers = [ maintainers.rycee ];
+
+  options.submoduleSupport = {
+    enable = mkOption {
+      type = types.bool;
+      default = false;
+      internal = true;
+      description = ''
+        Whether the Home Manager module system is used as a submodule
+        in, for example, NixOS or nix-darwin.
+      '';
+    };
+  };
+}