title: Generated grapql code just for the file extending the base grapql files
body: <p>I have split my monolithic app into two packages now. I will be subdividing further</p>
  <ol>
  <li>Core Models/Types - which have the base graphql models and some other TS interfaces. This is meant to be consumed by other web apps</li>
  <li>Web App - This uses core models/types. I have graphql files which extend the graphql files from the core model and  it defines queries, mutations, subscriptions, inputs, and outputs</li>
  </ol>
  <p>I now want to generate code for these. I want</p>
  <ol>
  <li>The core types have generated typescript models. This can be achieved easily by using <code>graphql-codegen</code></li>
  <li>The web app, which basically only generates the code for the extended graphql (should not include the core graphql code). How can this be done?</li>
  </ol>