title: Undefined class if class is in different namespace
body: <p>I have a yii2 project set up on phpstorm 8.0.3. My namespace structure is as follows:</p>
  
  <pre><code>backend\
    controllers
    models
    ...
  frontend\
    controllers
    models
    ..
  common\
    controllers
    models
  </code></pre>
  
  <p>I used composer on the project and have various dependancies installed in the vendor folder.</p>
  
  <p>Whenever I try to use a class that is not in the same base namespace (<code>frontend</code>, <code>backend</code> or <code>common</code>), I get an undefined class error. The odd thing is the namespace of the class is in autocomplete:</p>
  
  <p><a href="https://i.stack.imgur.com/OtW7f.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/OtW7f.png" alt="enter image description here"></a></p>
  
  <p>Another oddity is that the undefined class error disappears for some classes if the file with the defined class is opened.</p>