Correct resolution of unqualified System/FHIR Overlapping types

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](https://chat.fhir.org/#narrow/stream/179220-cql/topic/Resolving.20the.20name.20Quantity) 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.