title: "Required module not found" for module that exists in node_modules
body: <p>Some modules just seem to be invisible to Flow. For example I have <a href="https://www.npmjs.com/package/react-native-overlay" rel="nofollow">react-native-overlay</a> installed via npm into my node_modules directory but I get a whole bunch of errors like this from Flow:</p>
  
  <pre><code>[js/components/DatePickerOverlay.js:18
   18: let Overlay = require('react-native-overlay');
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ react-native-overlay. Required module not found
  </code></pre>
  
  <p>This module doesn't have types so it would be fine if I could just get Flow to ignore it entirely.</p>
  
  <p>Here's my .flowconfig (based on React Native's one):</p>
  
  <p><a href="https://gist.github.com/almost/20c6caf6d18d0e5c689f" rel="nofollow">https://gist.github.com/almost/20c6caf6d18d0e5c689f</a></p>
  
  <p>As you can see I'm on flow 0.20.1 and I have module.system=haste (as required by React Native)</p>
  
  <p>I tried adding a //$FlowIgnore comment to the import lines but then Flow complains about an unneeded ignore comment! I also tried creating a react-native-flow.js.flow file with a dummy export which seemed to work at first but then after a flow restart stopped working.</p>
  
  <p>Any ideas for how to either help Flow find this module or make it ignore the import line completely?</p>