title: webpack can not require variable ,the request of a dependency is an expression
body: <p>how to require a variable.
    i can work like this </p>
  
  <pre><code>var config={
  
      example1 : require("example1/main.js"),
  
      example2 : require("example2/main.js")
  
  }
  </code></pre>
  
  <p>when I  use like </p>
  
  <pre><code>var modules=["example1","example2"]
  
  _.each(modules,function(module){
  
      require(module+"/main.js")
  
  })
  </code></pre>
  
  <p>this error is "Cannot find module 'example1/main.js"</p>