Correct resolution of unqualified System/FHIR Overlapping types

[suyashkumar]

At the moment, iirc we will resolve an unqualified type that exists both in System and FHIR namespaces as the System one by default (e.g. types like Quantity, ValueSet, Ratio, etc), even when a using FHIR statement is present.

This isn't well documented yet, but according to the discussion here if only the using FHIR declaration exists explicitly we should resolve the FHIR type. If both the FHIR and system using statements are present, we should have it be an error.