title: Internal/External Header Separation
body: <p>I've built a library that's layed out like this:</p>
  
  <pre><code>lib/
     private_class_impl.cc
     private_class_decl.h
     public_class_impl.cc
  include/mylib/
               public_class_decl.h
  </code></pre>
  
  <p>My public class needs to have a private pointer to the private class.  </p>
  
  <p>Is there any way to expose <code>private_class_decl.h</code> to my declaration of <code>public_class_decl.h</code> without moving <code>private_class_decl.h</code> to <code>include/mylb/</code>?</p>