Use the auto-tagging feature for e.g. processors #405
Description
Similar to how twig does it: https://github.com/symfony/symfony/blob/master/src/Symfony/Bundle/TwigBundle/DependencyInjection/TwigExtension.php#L154
This line in the load function of EnqueueExtension.php should do it, but this is just one example:
$container->registerForAutoconfiguration(\PsrProcessor::class)->addTag('enqueue.client.processor');
It'd seem like a convenience for symfony4, with autowiring and autoconfigure.