title: CloudFirestorePlugin fails to compile on IOS
body: <p>I have a running app on Android, which is using a few plugins including cloud_firestore.
  Trying to compile for IOS it fails with </p>
  
  <p>Xcode's output:
  ↳</p>
  
  <pre><code> === BUILD TARGET Runner OF PROJECT Runner WITH CONFIGURATION Debug ===
      /ios/Runner/GeneratedPluginRegistrant.m:6:9: fatal error: 'cloud_firestore/CloudFirestorePlugin.h' file not found
      #import &lt;cloud_firestore/CloudFirestorePlugin.h&gt;
              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      1 error generated.
  </code></pre>
  
  <p>I know very little about IOS. Can someone tell me why this is happening?</p>