Add missing use statement to package event to fix install --no-dev

src/Composer/Installer/PackageEvent.php

@@ -18,6 +18,7 @@ use Composer\DependencyResolver\Operation\OperationInterface;
 use Composer\DependencyResolver\PolicyInterface;
 use Composer\DependencyResolver\Request;
 use Composer\Repository\CompositeRepository;
+use Composer\Repository\RepositorySet;
 
 /**
  * The Package Event.