title: Use already compiled Protobuf-classes in new .proto-files
body: <p>I have imported a library which contains classes generated by protobuf.</p>
  
  <p>I would like to send objects of this class as part of other protobuf messages. But <code>protoc</code> (obviously?) complains about missing definitions:</p>
  
  <blockquote>
    <p>Couldn't resolve reference to Type 'ExistingProtobufLibraryClass'.</p>
  </blockquote>
  
  <p>I could of course <em>copy</em> the existing .proto source from the source of the library, but then I suspect that protoc will generate <em>two</em> versions of this class, which would be quite annoying.</p>
  
  <p>Any ideas how to solve this?</p>