title: create-react-app importing modules as ES6 Modules when project is using CommonJS Modules
body: <p>I am trying to use CommonJS module format, with create-react-app (CRA). The documentation for CRA says:</p>
  
  <blockquote>
    <p>..Create React App can consume both CommonJS and ES modules. For Node.js
    compatibility, it is recommended that the main entry point is CommonJS...</p>
  </blockquote>
  
  <p>I am finding that when I import a module that offers ES6 module format (via is "module" property in "package.json"), then CRA uses this as the entry point for the module. </p>
  
  <p>Thus - even when my project is using CJS entirely, it then tries to import ES6 modules - and fails.</p>
  
  <p>So - <em>Is this a bug? Or, am I misunderstanding the intended behaviour of create-react-app?</em></p>
  
  <p><strong>Reproducing</strong></p>
  
  <p>I have reproduced this here: <a href="https://github.com/mjashanks/cra-test" rel="nofollow noreferrer">https://github.com/mjashanks/cra-test</a>. This project is a basic CRA skeleton, modified to used CJS.</p>
  
  <p>Additionally, I have added a "test-module", whose a package.json includes a "main" (CJS) and "module" (ES6) entry point.</p>
  
  <p>If we run <code>npm start</code>, the project fails to build, as the file "test-module/index.esm.js" is being used. (We can edit this file to use CJS format to make the project build and make this more clear). </p>
  
  <p>"test-module/index.cjs.js" is never used.</p>
  
  <p>Thanks :)</p>