title: Cannot load css file in webpack 4
body: <p>Given the following webpack 4 config:</p>
  
  <pre><code>const path = require('path')
  const webpack = require('webpack')
  const MiniCssExtractPlugin = require("mini-css-extract-plugin")
  const devMode = process.env.NODE_ENV !== 'production'
  const devFlagPlugin = new webpack.DefinePlugin({
    __DEV__: JSON.stringify(JSON.parse(process.env.DEBUG || 'false'))
  })
  
  module.exports = {
      mode: "development",
      entry: [
          'webpack-dev-server/client?http://0.0.0.0:3000',
          'webpack/hot/only-dev-server',
          "./app/app.js"
      ],
      output: {
          path: path.join(__dirname, "build"),
          filename: "bundle.js"
      },
      devtool: 'eval-source-map',
      resolve: {
          extensions: ['.js', '.jsx']
      },
      module: {
          rules: [
              { 
                  test: /\.jsx|\.js?$/, 
                  loader: 'babel-loader',
                  options: {
                      plugins: [
                          'react-hot-loader/babel'
                      ]
                  },
                  exclude: /node_modules/},
              {
                  test: /\.jsx|\.js$/,
                  exclude: /node_modules/,
                  use: [
                  'babel-loader?' + JSON.stringify({presets: ['env', 'react']}),
                  {
                      loader: 'eslint-loader'
                  }]
              },
              {
                  test: /\.css$/,
                  include: /stylesheets|node_modules/,
                  use: [ MiniCssExtractPlugin.loader, 'css-loader' ]
              },
              {
                  test: /\.scss$/,
                  include: /stylesheets/,
                  use: [ MiniCssExtractPlugin.loader, 'css-loader', 'sass-loader' ]
              },
              {
                  test: /\.(jpg|png|gif|eot|woff|ttf|svg)/,
                  loader: "file"
              }
          ]
      },
      devServer: {
          contentBase: "./build",
      },
      plugins: [
          new MiniCssExtractPlugin({
              filename: "[name].css",
              chunkFilename: "[id].css"
          }),
          devFlagPlugin,
          new webpack.NoEmitOnErrorsPlugin()
      ]
  
  };
  </code></pre>
  
  <p>I am trying to load a css file from /stylesheets/app.css in app.js:</p>
  
  <pre><code>require('./stylesheets/app.css')
  let routes = (
      &lt;div className="app"&gt;
          &lt;Router&gt;
              &lt;div&gt;hello&lt;/div&gt;
          &lt;/Router&gt;
      &lt;/div&gt;
  )
  
  render(routes, document.getElementById('root'))
  </code></pre>
  
  <p>The app.css file is defined as:</p>
  
  <pre><code>html {
      font-family: "Open Sans",Arial,sans-serif;
      font-size: 20px;
      height: 100%;
  }
  </code></pre>
  
  <p>But when I start the program with:</p>
  
  <pre><code>webpack-dev-server --host 0.0.0.0 --port 3000 --hot --progress --colors
  </code></pre>
  
  <p>The content has no style associated with it.</p>
  
  <p>Am I missing anything in the webpack config?</p>