title: Webpack check file exist and import in condition
body: <p>I use webpack in my react project.</p>
  
  <p>I want check file in 'foo/myfile' and if that file exist import in my component, or load from 'bar/myfile'. 
  for example:</p>
  
  <pre><code>let myModule;
  if(webpackCheckFileExist('foo/myfile.jsx')){
    myModule = require('foo/myfile.jsx');
  } else {
    myModule = require('bar/myfile.jsx');
  }
  </code></pre>
  
  <p>I try:</p>
  
  <pre><code>if(__webpack_modules__[require.resolveWeak("foo/myfile.jsx")]) {
    // do something when mod1 is available
  }
  </code></pre>
  
  <p>but if file dosnt exist - i catch webpack error</p>