Support resource key in services.yml

There's a use-case for this in 📌 Add symfony/config to core's dependencies for package_manager Closed: duplicate .

We have autowiring and autoconfiguration now; this would be useful so you could say

services:
    _defaults:
        autowire: true
        autoconfigure: true

    Drupal\module\EventSubscriber\:
        resource: 'src/EventSubscriber'

and then the container would automatically discover all event subscribers in that directory; to add a new one, you just add a new file and rebuild the container. We could even implement this sort of thing by default for modules so they don't necessarily have to write a services.yml at all!

However, there is quite a lot of Symfony code to port over. YamlFileLoader relies on the parent FileLoader class to do a bunch of discovery work here. The two options are:

1. Copy and refactor a number of methods from Symfony into YamlFileLoader
2. Implement ✨ Use native Symfony YamlLoader + Config Needs work

Symfony's container has diverged quite a lot from ours. I am leaning towards option 2 as it is simply a lot less work, and we would get all Symfony 3/4/5/6's features that we haven't implemented yet for free or almost free.

I've created an alternative proposal to resource that'll help us advance an equivalent more suitable in the Drupal context @ ✨ Directory based automatic service creation Needs review .

Perhaps it'll be easier to get something more Drupaly in.