lib: Better use the module type system in platform parsing

I need some module system types here so I can next fix meta-checks for
derivations. I'd like to use a "proper" record type here, but submodule
types seem overkill so holding off with ad-hoc stuff for now. In
practice, all I need for the next step are the `.check` functions so
this is good, especially as the submodule check function is shallow,
saving full inductive type-checking for a later step.