fix: handle case of implicit plugins/index.js files during migration (#22501)

* handle case of implicit index.js

* fix test error message

* fix test

scripts/gulp/tasks/gulpE2ETestScaffold.ts

@@ -37,7 +37,7 @@ export async function _e2eTestScaffold (cleanupEmpty = true) {
   await fs.writeFile(
     OUTPUT_PATH,
 `/* eslint-disable */
-// Auto-generated by ${path.basename(__filename)}
+// Auto-generated by ${path.basename(__filename)} (run yarn gulp e2eTestScaffold)
 export const fixtureDirs = [
 ${allDirs
 .map((dir) => `  '${path.basename(dir)}'`).join(',\n')}