permit core classes override

with this modification it is possible to write plugins that overrides most joomla core classes, for example JAccess, with a simple plugins like
```php
<?php
	defined('_JEXEC') or die;

	// include_once JPATH_SITE.'/overrides/access.php';
	JLoader::register('JAccess', JPATH_SITE.'/overrides/access.php');

?>
```