Automatically apply mockito codegen

Since this also uses `build_to: source` only the root package will auto apply the builder, and only if it mentions `mockito` as a direct dependency in the pubspec.

PiperOrigin-RevId: 336960382

build.yaml

@@ -4,3 +4,4 @@ builders:
     builder_factories: ["buildMocks"]
     build_extensions: {".dart": [".mocks.dart"]}
     build_to: source
+    auto_apply: dependents