[DIC] Move PrependExtensionInterface to the Extension namespace

src/Symfony/Component/DependencyInjection/Compiler/MergeExtensionConfigurationPass.php

@@ -12,6 +12,7 @@
 namespace Symfony\Component\DependencyInjection\Compiler;
 
 use Symfony\Component\DependencyInjection\ContainerBuilder;
+use Symfony\Component\DependencyInjection\Extension\PrependExtensionInterface;
 
 /**
  * Merges extension configs into the container builder

src/Symfony/Component/DependencyInjection/Extension/PrependExtensionInterface.php

@@ -9,7 +9,7 @@
  * file that was distributed with this source code.
  */
 
-namespace Symfony\Component\DependencyInjection\Compiler;
+namespace Symfony\Component\DependencyInjection\Extension;
 
 use Symfony\Component\DependencyInjection\ContainerBuilder;