Add support for resolving artifacts from a FhirPackageSource based on FHIR version.

[mmsmits]

We have already made a start in PR: #105

Todo:

Not all artifacts in the packages have a fhirVersion set in the index. We can fix that by adding logic that derives the FHIR Version from a package. The Car Factory already has some code for this in: NpmProject.FhirReleasesUsedInPackage

We will have to up the index version too when we add this logic