title: webpack.externals relative path causes Unexpected token
body: <p>when I add a relative path to <code>webpack.externals</code> array, the output file contains: <code>module.exports = ../path;</code> which causes the error: Unexpected token.</p>
  <pre><code>//webpack.config.js
  {
   externals: ['../path']
  }
  </code></pre>
  <pre><code>//input.ts
  import something from '../path';
  </code></pre>
  <pre><code>//output.js
  module.exports = ../path;
  </code></pre>