Move SDC specific logic out of ClientsideConversionTrait into component source plugins

First pass at doing the simplest thing possible here, also opened #3497203: Throw exceptions instead of returning tuples for validation errors as passing the violations around is kinda messy when we could just throw exceptions.

I also feel like the source plugins are now doing way too much, SingleDirectoryComponent has 11 dependencies and 41 use statements and we're not really close to finishing it.

reviewing

I also feel like the source plugins are now doing way too much, SingleDirectoryComponent has 11 dependencies and 41 use statements and we're not really close to finishing it.

I agree that is a lot. But also seems like this issue doesn't make it much works. Adds 1 dependency, entity type manager, but maybe the would removed in #3473336: The UI only handles resolved props values, hence complex widgets do not show the current value in props form (e.g. image with Media Library Widget) which is linked from \Drupal\experience_builder\Plugin\ExperienceBuilder\ComponentSource\SingleDirectoryComponent::findTargetForProps

also seems like findTargetForProps could be replace by generic getTargetForSrc(string $src, string $type) somewhere but maybe it will go away completely.

I had 1 question on the MR but think it is find the way it is

Thanks @longwave!

Nice incremental improvement 👍

@larowlan's post-merge MR comment will be addressed in #3497203: Throw exceptions instead of returning tuples for validation errors 👍