lib.options.mkPackageOption: use lib.showAttrPath

Make use of `lib.showAttrPath` instead of manually doing `concatStringsSep "."`.

This means edge-cases such as the attr-path including names that are not
valid nix identifiers will be handled better.

See:
- https://nix.dev/manual/nix/2.26/language/identifiers
- https://nixos.org/manual/nixpkgs/unstable/#function-library-lib.attrsets.showAttrPath

lib/tests/modules/declare-mkPackageOption.nix

@@ -57,5 +57,19 @@ in
         };
       in
       lib.mkPackageOption myPkgs "hello" { };
+
+    packageInvalidIdentifier =
+      let
+        myPkgs."123"."with\"quote" = { inherit (pkgs) hello; };
+      in
+      lib.mkPackageOption myPkgs [ "123" "with\"quote" "hello" ] { };
+
+    packageInvalidIdentifierExample = lib.mkPackageOption pkgs "hello" {
+      example = [
+        "123"
+        "with\"quote"
+        "hello"
+      ];
+    };
   };
 }