title: List all dependencies used by a particular dependency in a Podfile
body: <p>For example, my Podfile has</p>
  
  <p>pod x<br>
     pod y<br>
     pod z  </p>
  
  <p>When I run <code>pod install</code>, it installs 'a', 'b', and 'c', in addition to 'x', 'y', and 'z'.
  If I want to find out where 'a' comes from, (is it from 'x' or 'y' or 'z', or do they share), how do I find out?</p>