Not working with css-loader options.modules

Testing with webpack v4.0.1 :
```javascript
{
  test: /\.css$/,
  use: [
          MiniCssExtractPlugin.loader,
          { loader: 'css-loader', options: { modules: true } }
  ]
}
```

When requiring css file, it always get empty object:

```javascript
const styles = require('xxx.css'); // always get {} even it is not.
```

