Fix overlapped Identity name definition

Fix "Cannot use eZ\Publish\SPI\User\Identity as Identity because the name is already in use in /.../vendor/ezsystems/ezpublish-kernel/eZ/Publish/Core/MVC/Symfony/Security/User/HashGenerator.php on line 13"

Identity is already defined in namespace `eZ\Publish\Core\MVC\Symfony\Security\User` @see `eZ\Publish\Core\MVC\Symfony\Security\User\Identity`.