What is the best-practice for structuring a collection of MC packages
containing for platform-independent portions?

For example, I currently have "Magma client' which has code specific
to Squeak.  So I made a new package:

  Magma client-Squeak

And then renamed the method-extension category from "*magma client" to
"*magma client-squeak".

Unfortunately, PackageInfo considers #isYourClassExtension: to be true
even if only the "core" package name matches the _prefix_ of the
extension category name ("*magma client-squeak"), causing those
extensions end up in two packages; "Magma client" as well as "Magma
client-Squeak".