I've missed the previous pitch, but now having a closer look at this, I'm in favor in general, even though the implementation details seem a bit convoluted. I haven't seen similar features implemented in other languages, but I also haven't stumbled upon this exact set of issues in other languages. Are "peer dependencies" in NPM similar in any way?
I personally would prefer JSON to YAML. Even though JSON doesn't allow comments, at least it doesn't have weird type inference issues (such as no and yes values parsed as a boolean, not a string). Nevertheless, I realise that the YAML type inference issues are unlikely in this particular case, and if the presence of comments is required, I have no better alternative to propose (even though I wish that configuration languages like Dhall got some wider adoption). And as this is already present in 5.3 nightlies, the YAML ship has probably sailed.
Another thing I'd love to see from proposals and pitches going forward is to clarify the impact on Foundation and other core libraries if such is present. For example, I'd like to know if Foundation and Dispatch extensions for Combine are going to be moved to overlay modules. These implicitly imported extensions are a major pain when dealing with libraries such as OpenCombine. @broadway_lamb do I understand correctly that moving those extensions to an overlay module according to this proposal would solve those problems?