Support fully defining contained resources using caret rules

[cmoesel]

It should be possible to fully define a contained resource, element-by-element, using caret rules in a non-Instance FSH definition. E.g.:

ValueSet: ExampleValueset
Id: example-valueset
* ^contained.resourceType = "CodeSystem"
* ^contained.id = "example-codesystem"
* ^contained.url = "http://example.org/codesystem"
* ^contained.version = "1.0.0"
* ^contained.content = #complete
* ^contained.concept[0].code = #example-code-1
* ^contained.concept[0].display = "Example Code 1"
* ^url = "http://example.org/valueset"
* ^version = "1.0.0"
* ^status = #active

Currently this results in an error because SUSHI does not recognize the path ^contained.url since it is not in the Resource datatype. Ideally, SUSHI should look at the assigned resourceType and use that type when resolving paths in the contained resource FSH. Note that this does work correctly when assigning contained resources in an Instance definition.

See: https://chat.fhir.org/#narrow/stream/215610-shorthand/topic/Contained.20code.20system.20in.20the.20value.20set/near/407685349