Support source plugin configs with plugin YAML

There are many quite complex source plugins at this point. It seems that it's not supported to have configs with plugin YAML, only project YAML or element YAML. This makes documentable defaults basically impossible unless I'm missing something. Let's ensure source plugins behave same way as build element plugins with regards to configs.

Start by comparing how source plugins and build element plugins handle configuration in plugin YAML, then trace the existing project YAML and element YAML behavior. Done means source plugins support equivalent configs and allow their defaults to be documented.

Written by the indexing model from the issue text.