[cryptonector]

Well, for one thing a dependence on an interface could not have a hash to bind the provider(s), but one could have a dependence on an interface and also associated dependencies on one-of-N providers of the interface, then the latter could have hashes.

Basically you need a way to indicate "this package is an interface and requires providers of it" and also you need a way to indicate which packages are the associated providers (either as attributes of the interface PURLs, as attributes of the provider PURLs, or both).