title: copy-webpack-plugin doesn't copy files
body: <p>I try to just copy files to check simple webpack config. So I stuck trying to make copy-webpack-plugin to work - nothing happens: no copied files, no errors, nothing</p>
  
  <p>Common config (webpack.common.js):</p>
  
  <pre><code>const path = require('path');
  
  const CopyWebpackPlugin = require('copy-webpack-plugin');
  
  const postCssPlugin = [
    require('postcss-import'),
    require('postcss-nested'),
    require('postcss-simple-vars'),
    require('autoprefixer')({
      browsers: [
        'last 3 versions',
        'android 4.2'
      ]
    })
  ];
  
  module.exports = {
    context: path.resolve(__dirname, '../src'),
    entry: [
      'babel-polyfill',
      path.resolve(__dirname, '../src/index.js')
    ],
    output: {
      path: path.resolve(__dirname, '../dist/js'),
      publicPath: '',
      filename: 'app.js'
    },
    resolve: {
      extensions: ['.jsx', '.js', '.json']
    },
    module: {
      rules: [
        {
          test: /\.(js|jsx)$/,
          exclude: /node_modules/,
          loader: 'babel-loader'
        },
        {
          test: /\.p?css$/,
          use: [
            'style-loader',
            'css-loader',
            {
              loader: 'postcss-loader',
              options: {
                ident: 'postcss',
                plugins: postCssPlugin
              }
            }
          ]
        }
      ]
    },
    plugins: [
      new CopyWebpackPlugin([
        {
          from: path.resolve(__dirname, '../src/assets/**/*'),
          to: path.resolve(__dirname, '../dist/assets'),
          flatten: true
        }
      ])
    ],
    stats: {
      modules: true,
      warnings: false,
      version: true,
      timings: true,
      performance: false,
      hash: true,
      errors: true,
      errorDetails: true,
      colors: true,
      builtAt: true
    }
  };
  </code></pre>
  
  <p>webpack.prod.js:</p>
  
  <pre><code>const commonWebpackConfig = require('./webpack.common.js');
  
  const UglifyJsWebpackPlugin = require('uglifyjs-webpack-plugin');
  
  module.exports = Object.assign(commonWebpackConfig, {
    mode: 'production',
    plugins: [
      new UglifyJsWebpackPlugin({
        sourceMap: true
      })
    ]
  });
  </code></pre>
  
  <p>And build starting file build.js:</p>
  
  <pre><code>const webpack = require('webpack');
  
  const webpackProdConfig = require('./webpack.config/webpack.prod.js');
  
  webpack(webpackProdConfig, (err, stats) =&gt; {
    if (err || stats.hasErrors()) {
      console.error(err.stack || err);
    }
  
    console.log('Successfully compiled!');
  });
  </code></pre>
  
  <p>So could anyone figure out why doesn't it work and where am I wrong?
  copy-webpack-plugin: 4.5.2
  node: 9.1.0
  npm: 6.3.0
  windows: 10</p>
  
  <p>Addition - folder structure:</p>
  
  <p><a href="https://i.stack.imgur.com/TeSZj.png" rel="noreferrer"><img src="https://i.stack.imgur.com/TeSZj.png" alt="enter image description here"></a></p>