title: how to declare a external package in package.js
body: <h1>situation:</h1>
  <ol>
  <li>I have three packages: A, B, and C.</li>
  <li>All of those packages have a common dependency package: D.</li>
  <li>And I have a project which depends on package A, B, C , and D.</li>
  <li>package D is a big package (size &gt; 500KB)</li>
  <li>When I run <strong>npm run build</strong> in my project's root directory. Then the D package
  which is very big will be process 4 times, and make a very big bundle for my project!</li>
  </ol>
  <h1>question:</h1>
  <p>Is there any way to make my project's bundle smaller ? I just want to process D package one time.</p>