Some related discussion in Pitch: Fully qualified name syntax
AFAIK there isn't anything currently available that will do what you're looking for. The only thing I can think of that comes close is having multiple targets (importable modules) in one package. However there is no parent/child relationship.
I'm expecting "Sub"/"Child" Packages will be added at some point. I can't speak to how that's prioritized or whether there is progress being made toward a full proposal.