title: Webpack not being recognized after updates
body: <p>In my laravel app, after updating to <a href="https://github.com/JeffreyWay/laravel-mix/releases/tag/v4.0.0" rel="nofollow noreferrer">laravel-mix v4</a>, when I run <code>npm run watch</code>, I get the error:</p>
  
  <blockquote>
    <p>webpack not installed, </p>
    
    <p>consider installing it using  npm install
    --save-dev webpack</p>
  </blockquote>
  
  <p>Even when Webpack v4.33.0 is clearly installed &amp; available in my node_modules. Here is my package.json:</p>
  
  <pre><code>{
    "private": true,
    "scripts": {
      "dev": "npm run development",
      "development": "cross-env NODE_ENV=development node_modules/webpack/bin/webpack.js --progress --hide-modules --config=node_modules/laravel-mix/setup/webpack.config.js",
      "watch": "npm run development -- --watch",
      "watch-poll": "npm run watch -- --watch-poll",
      "hot": "cross-env NODE_ENV=development node_modules/webpack-dev-server/bin/webpack-dev-server.js --inline --hot --config=node_modules/laravel-mix/setup/webpack.config.js",
      "prod": "npm run production",
      "production": "cross-env NODE_ENV=production node_modules/webpack/bin/webpack.js --no-progress --hide-modules --config=node_modules/laravel-mix/setup/webpack.config.js"
    },
    "devDependencies": {
      ...
      "laravel-mix": "^4.0.16",
      "webpack": "^4.33.0"
    }
  }
  </code></pre>
  
  <p>Here is my webpack.mix.js:</p>
  
  <pre><code>mix.js('resources/assets/js/app.js','public/js/app.js')
  .browserSync({
      proxy: 'app.test',
      notify: false,
      open: true,
      port: 8080
  })
  .options({
      extractVueStyles: OUTPUTCSS + 'app.css'
  });
  </code></pre>
  
  <p>This wasn't an issue prior to updating my <code>laravel-mix</code> to v4, what seems to be the issue?</p>
  
  <p><strong>--- UPDATE ---</strong></p>
  
  <p>Here is the <code>npm debug log</code>:</p>
  
  <pre><code>0 info it worked if it ends with ok
  1 verbose cli [ '/usr/local/bin/node', '/usr/local/bin/npm', 'run', 'watch' ]
  2 info using npm@6.2.0
  3 info using node@v10.9.0
  4 verbose run-script [ 'prewatch', 'watch', 'postwatch' ]
  5 info lifecycle @~prewatch: @
  6 info lifecycle @~watch: @
  7 verbose lifecycle @~watch: unsafe-perm in lifecycle true
  8 verbose lifecycle @~watch: PATH: /usr/local/lib/node_modules/npm/node_modules/npm-lifecycle/node-gyp-bin:/Users/main_user/projects/test_project/node_modules/.bin:/usr/local/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/opt/X11/bin
  9 verbose lifecycle @~watch: CWD: /Users/main_user/projects/test_project
  10 silly lifecycle @~watch: Args: [ '-c', 'npm run development -- --watch' ]
  11 silly lifecycle @~watch: Returned: code: 1  signal: null
  12 info lifecycle @~watch: Failed to exec watch script
  13 verbose stack Error: @ watch: `npm run development -- --watch`
  13 verbose stack Exit status 1
  13 verbose stack     at EventEmitter.&lt;anonymous&gt; (/usr/local/lib/node_modules/npm/node_modules/npm-lifecycle/index.js:304:16)
  13 verbose stack     at EventEmitter.emit (events.js:182:13)
  13 verbose stack     at ChildProcess.&lt;anonymous&gt; (/usr/local/lib/node_modules/npm/node_modules/npm-lifecycle/lib/spawn.js:55:14)
  13 verbose stack     at ChildProcess.emit (events.js:182:13)
  13 verbose stack     at maybeClose (internal/child_process.js:961:16)
  13 verbose stack     at Process.ChildProcess._handle.onexit (internal/child_process.js:250:5)
  14 verbose pkgid @
  15 verbose cwd /Users/main_user/projects/test_project
  16 verbose Darwin 18.6.0
  17 verbose argv "/usr/local/bin/node" "/usr/local/bin/npm" "run" "watch"
  18 verbose node v10.9.0
  19 verbose npm  v6.2.0
  20 error code ELIFECYCLE
  21 error errno 1
  22 error @ watch: `npm run development -- --watch`
  22 error Exit status 1
  23 error Failed at the @ watch script.
  23 error This is probably not a problem with npm. There is likely additional logging output above.
  24 verbose exit [ 1, true ]
  </code></pre>