Partially revert "lib/modules: Drop mkStrict and mkFixStrictness"

mkFixStrictness was never properly deprecated and should only be
removed after warning for some time.

This partially reverts commit 8fb9984690.

lib/modules.nix

@@ -713,6 +713,10 @@ rec {
   mkForce = mkOverride 50;
   mkVMOverride = mkOverride 10; # used by ‘nixos-rebuild build-vm’
 
+  mkStrict = builtins.trace "`mkStrict' is obsolete; use `mkOverride 0' instead." (mkOverride 0);
+
+  mkFixStrictness = id; # obsolete, no-op
+
   mkOrder = priority: content:
     { _type = "order";
       inherit priority content;