That feels like a semantic difference. Regardless of the name we assign to it, remapping or following the same protocol as the parent, the methodology allows the goal to be achieved. And, at least in the context of this discussion, I'd consider git submodule to be performing many of the same functions as a package manager. Therefore, I do see at as something for SwiftPM to use as a blueprint.
If I had to assign a name or description to what I'm proposing I'd describe it is expanding relative URLs. 
All I can say is my team has done this for years and never had a problem in mixed environments; of course using submodules and not Cocoapods or Carthage or another higher level manager. But based on this feedback it is possible we are the exception and not the rule.