title: Make Webpack aliases not apply in node_modules
body: <p>I made an alias in webpack.config.js, but I want that alias to apply only in my app code, not in node_modules.</p>
  
  <p>Here's the alias:</p>
  
  <pre><code>alias: {
    'jss': path.resolve('path', 'to', 'custom', 'jss')
  }
  </code></pre>
  
  <p>I would like something similar to <code>rules</code> (or <code>loaders</code> in Webpack v1) where we can write</p>
  
  <pre><code>exclude: /node_modules/
  </code></pre>
  
  <p>so that the rules/loaders don't apply to node_modules.</p>
  
  <p>How can we do this with aliases?</p>
  
  <p>For example, there's a package inside <code>node_modules</code> that does <code>require('jss')</code>, and I want it to get the actual <code>jss</code> package, not my customized module.</p>