SUSHI does not recognize FML files when processing sushi-config.yaml / generating IG JSON

The FHIR publishing system supports defining structure maps via [FML](https://www.hl7.org/fhir/mapping-language.html) files w/ [annotated metadata](https://www.hl7.org/fhir/mapping-language.html#metadata).

When SUSHI reads in predefined resources, it only reads in XML and JSON. As a result, if an author refers to a StructureMap in their sushi-config.yaml, and that StructureMap is defined using FML, SUSHI will think that StructureMap doesn't exist and issue an error. Instead, SUSHI needs to read in FML files and at least understand enough to validate that the StructureMap exists when processing sushi-config.yaml or generating IG JSON.

See: https://chat.fhir.org/#narrow/stream/179252-IG-creation/topic/Sushi.20Support/near/372771763

Trace how predefined resources are loaded when processing sushi-config.yaml and when generating IG JSON, then inspect the existing XML and JSON handling. Determine where a referenced StructureMap is rejected when it is defined in FML. Done means FML-defined StructureMaps are recognized and validated in both workflows without breaking existing resource loading.

Written by the indexing model from the issue text.