title: Webpack 4: mini-css-extract-plugin + sass-loader + splitChunks
body: <p>I've the follow example configuration to use mini-css-extract-plugin with Webpack 4:</p>
  
  <pre><code>entry: {
     a: ['./js/a.js', './scss/a.scss'],
     b: ['./js/b.js', './scss/b.scss']
  },
  module: {
      rules: [
         [...],
         {
          test: /\.(css|sass|scss)$/,
          use: [
              MiniCssExtractPlugin.loader,
              {
                  loader: 'css-loader',
                  options: {
                      importLoaders: 2,
                      sourceMap: true
                  }
              },
              {
                  loader: 'postcss-loader',
                  options: {
                      plugins: () =&gt; [
                          require('autoprefixer')
                      ],
                      sourceMap: true
                  }
              },
              {
                  loader: 'sass-loader',
                  options: {
                      sourceMap: true
                  }
              }
          ]
  },
  optimization: {
      splitChunks: {
          cacheGroups: {
              js: {
                  test: /\.js$/,
                  name: "commons",
                  chunks: "all",
                  minChunks: 7,
              },
              css: {
                  test: /\.(css|sass|scss)$/,
                  name: "commons",
                  chunks: "all",
                  minChunks: 2,
              }
          }
      }
  },
  plugins: [
      new MiniCssExtractPlugin({
          filename: "dist/[name].css",
      }),
  ]
  </code></pre>
  
  <p>And the following sass files:</p>
  
  <pre><code>// a.scss
  @import 'libA.scss';
  @import 'libB.css';
  [...] 
  
  // b.scss
  @import 'libA.scss';
  @import 'libB.css';
  [...]
  </code></pre>
  
  <p>When I run webpack <code>libB.css</code> is inserted in in <code>commons.css</code> bundle while <code>libA.scss</code> not.</p>
  
  <p>In general every import of <code>.css</code> file get processed by splitChunks option (only if extension <code>css</code> is specified in the name) while sass import not.</p>
  
  <p>I have a project with multiple sass entry point and many @import of sass component and I'd like to create a common bundle with shared sass modules.</p>