[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/functional/fixtures/ext/foo/bar/ext.php

@@ -1,6 +1,8 @@
 <?php
 
-class phpbb_ext_foo_bar_ext extends \phpbb\extension\base
+namespace foo\bar;
+
+class ext extends \phpbb\extension\base
 {
 
 }