title: Rollup plugin postcss does not inject css imported from node_modules
body: <p>Here is my rollup config</p>
  <pre><code>// rollup.config.js
  const postcss = require('rollup-plugin-postcss');
  const autoprefixer = require('autoprefixer');
  
  module.exports = {
    rollup(config, _options) {
      config.plugins.push(
        postcss({
          plugins: [
            autoprefixer(),
          ],
          extensions: ['.css'],
          modules: false,
          extract: false,
        }),
      );
      return config;
    },
  };
  </code></pre>
  <p>So if I import css file local from a relative path, it gets injected but I import from node_modules it doesn't</p>
  <pre><code>import React from 'react';
  import { toast } from 'react-toastify';
  import 'react-toastify/dist/ReactToastify.css';
  // The following works if I copy the file locally
  // import './ReactToastify.css';
  </code></pre>
  <p>What am I doing wrong here?</p>