Edit: redaction
You can take what you've said, replace "JSON" with "XML" and you've got an explanation for Maven and all the other Java tools with a ridiculous amount of logic shoved into some generic markup format that was never intended for it. History has shown that it sucks to deal with. XML was the hot, "obvious", "everybody knows it", format of its day. I agree that JSON is, in most uses, a hell of a lot better, but, like XML it is a terrible place to be defining logic.
It really needs to be something that is actually designed to encode logic, not a data structure format. A custom DSL is a decent, and probably very easy (for users) idea, but if the author was really not interested in the design and debugging of such a thing then you could use LUA or some other embedded language.
I think I even mentioned embedded JS or Lua in some other comment.