Sub-package auto-export

Something that I see a lot is packages that would work really well together, but can't directly import the package in question in order to remain pure (ex flutter widget libraries supporting the mix package)

I propose some mechanism to conditionally federate packages based on what packages are imported in the application.

For instance, my package would like to generate utilities for package x
And let's say I made a separate package for it. The problem is, users don't know that package exists. And what if I support a ton of packages? How many mini packages are users expected to find? Across multiple packages?
If my package simply included the sub packages when x is depended on, then users just _get_ first party support automatically.

It has a ton of possibilities.
We could easily expand on conditional imports and exports if we wanted to keep it in-package, which would be even better.