title: Can't use plugin module with Cloud Composer
body: <p>I'm trying to use the Cloud Composer to run my workflow. I wanted to use "GoogleCloudStorageToGoogleCloudStorageOperator" operator which is available from Apache Airflow v1.10, but not being supported in current cloud composer (it supports only Apache Airflow v1.9 for now (2019/01/16)). </p>
  
  <p>Following the guidance of the <a href="https://gweb-cloudblog-slim.appspot.com/blog/products/data-analytics/how-to-transfer-bigquery-tables-between-locations-with-cloud-composer" rel="nofollow noreferrer">Google's blog post</a>, I added the operator to a cloud composer environment by myself, and it worked well until a few days ago.</p>
  
  <p>However, for now, when I tried to create a new cloud composer env and to deploy the same DAG that worked well previously, I got a following error message on the Airflow Web UI. And DAG is failed.</p>
  
  <pre><code>Broken DAG: [/home/airflow/gcs/dags/xxx.py] Relationships can only be set between Operators; received GoogleCloudStorageToGoogleCloudStorageOperator
  </code></pre>
  
  <p>I couldn't understand why this error occurred even if I used the same code and followed the same procedure to deploy my DAG to the cloud composer.</p>
  
  <p>I appreciate for those who give me any advice to solve this problem.</p>