Drupal multisite compatibility.

[dpagini]

Feature request

From #phpstan Drupal slack:

In my project, I am using Drupal multisite, and have modules defined at docroot/sites/mysite/modules/____.

So let's say I have 2 modules... docroot/sites/mysite/modules/mySiteModule1 and docroot/sites/mysite/modules/mySiteModule2 - in mySiteModule2 I am referencing a class defined in mySiteModule1, but PHPStan is telling me that class doesn't exist. I'm referencing a class like use Drupal\mySiteModule1\Controller\MyBaseController.