nixos/modules: Allow options to be coerced to a string for convenience

lib/modules.nix

@@ -587,6 +587,8 @@ rec {
         definitions = map (def: def.value) res.defsFinal;
         files = map (def: def.file) res.defsFinal;
         inherit (res) isDefined;
+        # This allows options to be correctly displayed using `${options.path.to.it}`
+        __toString = _: showOption loc;
       };
 
   # Merge definitions of a value of a given type.