Validator based on a directory does not check local file in profile based slices

[bvdh]

When validating a set of files in a directory, errors are generated for slices based on profiles as the validator does not load these files using the FHIRpath resolve() method.
The validator IWorkerContext should be extended to look for references in the local files as well. This is easy as these are already loaded.

The branch https://github.com/bvdh/org.hl7.fhir.core/tree/include-local-files contains an update to InstanceValidator.java that checks the context for the file and returns it when found.

Please merge this into the validator.

[grahamegrieve]

This needs a test case, I think