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
diff --git a/build.yaml b/build.yaml
index 755c286..3945a14 100644
--- a/build.yaml
+++ b/build.yaml
@@ -4,3 +4,4 @@
     builder_factories: ["buildMocks"]
     build_extensions: {".dart": [".mocks.dart"]}
     build_to: source
+    auto_apply: dependents