Add code for Z6908; upgrade Z6903 & Z6907 to handle differing enum representations
These are the implementations for:
- Z6803/'Same Wikidata statement'
- Z6807/'Same Wikidata claim'
- Z6808/'Same Wikidata reference'
"handle differing enum representations" refers to basic enums, not "lightweight" enums. This MR allows us to recognize that, for example, these are the same:
- "Z6003K7": {"Z1K1": "Z6020", "Z6020K1": "Z6021"}
- "Z6003K7": "Z6021"
Also, Add missing keys to some Z6803 test inputs
Bug: T407700
Bug: T414287