Fix Some (Not All) Profile-on-Profile Errors
Fix Some (Not All) Profile-on-Profile Errors

Fixes the following issues when mapping to an existing profile:
- Correctly determines base id in elements
- If extension already exists, modifies it (no duplicates)

Also fixes one discovered bug with creating a differential for the code element when profiling on Basic.

standardhealth/shr-fhir-export,standardhealth/shr-fhir-export