isDeprecatedConfigDirectorySettingUsed() does not define $class_loader in case settings.php will need it

Hm, core's Drupal\Core\Site\Settings class does not provide the class loader to settings.php either when it includes it for settings handling. Why would we need to?

Ah and its done intentionally.

   * @param \Composer\Autoload\ClassLoader $class_loader
   *   The class loader that is used for this request. Passed by reference and
   *   exposed to the local scope of settings.php, so as to allow it to be
   *   decorated with Symfony's ApcClassLoader, for example.

So the question is if this is the best way to get the class loader that is in the state normally passed into settings.php.

index.php has

$autoloader = require_once 'autoload.php';
$kernel = new DrupalKernel('prod', $autoloader);

which is then saved as classLoader in the kernel which is passed into the settings initializer.

Looks good, let's get in!