title: Yarn add does not install dependencies
body: <p>I am using the latest version of stable <code>yarn 1.3.2</code>. Everytime I run <code>yarn add &lt;package&gt;</code> it adds it to my <code>package.json</code>and <code>yarn.lock</code> but the <code>node_modules</code> does not get installed.</p>
  
  <p>I have tried <code>yarn install --force</code> and still no result. The only way to get it to work is to delete my <code>node_modules</code> folder and run <code>yarn install</code>. This is annoying to do for every package I add mid project.</p>
  
  <p>Has anyone faced this issue?</p>