The culprit seems to be specifically StructOnScope.h, which is indirectly included by EngineMinimal.h, but not CoreMinimal.h. That file does mention the UPackage class. I have not looked further though.
The culprit seems to be specifically StructOnScope.h, which is indirectly included by EngineMinimal.h, but not CoreMinimal.h. That file does mention the UPackage class. I have not looked further though.