[ticket/11700] Fix extension loading with namespaces

class loader now expects all classes to be prefixed with a backslash
when resolving paths

PHPBB3-11700

tests/controller/ext/foo/config/services.yml

@@ -1,3 +1,3 @@
 services:
     foo.controller:
-        class: phpbb_ext_foo_controller
+        class: foo\controller

tests/controller/ext/foo/controller.php

@@ -1,8 +1,10 @@
 <?php
 
+namespace foo;
+
 use Symfony\Component\HttpFoundation\Response;
 
-class phpbb_ext_foo_controller
+class controller
 {
 	/**
 	* Handle method