Fix conflicting import

"Extension" is already in use in the "PDepend\DependencyInjection" namespace.

```
$ php -r '
require "vendor/autoload.php";
require "src/main/php/PDepend/DependencyInjection/Extension.php";
require "src/main/php/PDepend/DependencyInjection/PdependExtension.php";
'

PHP Fatal error:  Cannot use Symfony\Component\DependencyInjection\Extension\Extension as Extension because the name is already in use in [...]/src/main/php/PDepend/DependencyInjection/PdependExtension.php on line 48
PHP Stack trace:
PHP   1. {main}() Command line code:0
```