Reuse document indexes in D&D normalization

internal/modules/dnd/shared/unit_refs.go

@@ -77,11 +77,11 @@ func (ref UnitRef) MarshalJSON() ([]byte, error) {
 	return json.Marshal(ref.String())
 }
 
-func ResolveUnitID(doc *source.SourceDocument, field string, ref UnitRef) (int, error) {
+func ResolveUnitID(index source.DocumentIndex, field string, ref UnitRef) (int, error) {
 	if ref.value <= 0 {
 		return 0, fmt.Errorf("%s must be positive", field)
 	}
-	if _, ok := source.UnitIndex(doc, ref.value); !ok {
+	if _, ok := index.Position(ref.value); !ok {
 		return 0, fmt.Errorf("%s %d was not found", field, ref.value)
 	}
 	return ref.value, nil