title: yarn start fails after jhipster 5.0.1 upgrade
body: <p>I've generated a JHipster / React app and after upgrading JHipster to 5.0.1 (from 5.0.0-beta.1) and running <em>jhipster upgrade</em>, I get the following when I run <em>yarn start</em>:</p>
  
  <pre><code>WARNING in configuration
  The 'mode' option has not been set, webpack will fallback to 'production' for this value. Set 'mode' option to 'development' or 'production' to enable defaults for each environment.
  You can also set it to 'none' to disable any default behavior. Learn more: https://webpack.js.org/concepts/mode/
  
  ERROR in multi (webpack)-dev-server/client?http://localhost:8081 ./src
  Module not found: Error: Can't resolve './src' in '/Users/yvon/git/ods-jhipster-sns'
   @ multi (webpack)-dev-server/client?http://localhost:8081 ./src
  ℹ ｢wdm｣: Failed to compile.
  </code></pre>
  
  <p>I tried generating a branch new jhipster app using the 5.0.1 generator, but <em>yarn start</em> fails at exactly the same spot.</p>
  
  <p>Here's the relevant part of <strong>package.json</strong>:</p>
  
  <pre><code>  "scripts": {
  "precommit": "lint-staged",
  "prettier:format": "yarn prettier --write 'src/**/*.{ts,tsx,css,scss}'",
  "lint": "tslint --project tsconfig.json -e 'node_modules/**'",
  "lint:fix": "yarn run lint --fix",
  "cleanup": "rimraf target/www",
  "start": "yarn run webpack:dev",
  "test": "yarn run lint &amp;&amp; jest --coverage --logHeapUsage -w=2 --config src/test/javascript/jest.conf.js",
  "test:watch": "yarn test --watch",
  "webpack:dev": "yarn run webpack-dev-server --config webpack/webpack.dev.js --progress --inline --profile --port=9060",
  "webpack:build:main": "yarn run webpack --config webpack/webpack.dev.js --progress --profile",
  "webpack:build": "yarn run cleanup &amp;&amp; yarn run webpack:build:main",
  "webpack:prod:main": "yarn run webpack --config webpack/webpack.prod.js --profile",
  "webpack:prod": "yarn run cleanup &amp;&amp; yarn run webpack:prod:main",
  "webpack:test": "yarn run test",
  "webpack-dev-server": "node --max_old_space_size=4096 node_modules/webpack-dev-server/bin/webpack-dev-server.js",
  "webpack": "node --max_old_space_size=4096 node_modules/webpack/bin/webpack.js"},
  </code></pre>
  
  <p>and <strong>webpack-dev.js</strong>:</p>
  
  <pre><code>const webpack = require('webpack');
  const writeFilePlugin = require('write-file-webpack-plugin');
  const webpackMerge = require('webpack-merge');
  const BrowserSyncPlugin = require('browser-sync-webpack-plugin');
  const WebpackNotifierPlugin = require('webpack-notifier');
  const path = require('path');
  
  const utils = require('./utils.js');
  const commonConfig = require('./webpack.common.js');
  
  const ENV = 'development';
  
  module.exports = webpackMerge(commonConfig({ env: ENV }), {
    devtool: 'cheap-module-source-map', // https://reactjs.org/docs/cross-origin-errors.html
    mode: 'development',
    entry: [
      'react-hot-loader/patch',
      './src/main/webapp/app/index'
    ],
  </code></pre>
  
  <p>Is JHipster/React 5.0.1 broken, or am I missing something obvious?</p>