What's the difference between $$description and description? They seem to both be used in this file interchangeably. I think I knew at one point. Something about line breaks? Yes, that's what it is. Also, SchemaStore/schemastore#4139 uses title everywhere, while we use it only on the outermost item. From https://json-schema.org/understanding-json-schema/reference/annotations, it seems to me title is the one you normally want, with description needed for longer pieces of information. Though, the SchemaStore docs say to use description. No idea, let's just keep matching styles for now.

Pretty sure we can't validate that include-groups point to an existing group.

I'm fairly sure this is right. There's no generic mechanism in JSON Schema for referring to the literal keys of an input object as far as I know.

Added a dynamic check for this case.

Circular deps could probably be done too but I'm tempted to wait till we are 3.9+, since IIRC this is easy with graphlib.