Use active_types in ReflectEnum::get_type_registration.

Instead of `active_fields`. This makes it match `ReflectStruct`, and
avoids redundant calls within the generated `register_type_dependencies`
when the same type is used in multiple enum variant fields.

Author: nnethercote

crates/bevy_reflect/derive/src/derive_data.rs

@@ -879,7 +879,7 @@ impl<'a> ReflectEnum<'a> {
             self.meta(),
             where_clause_options,
             None,
-            Some(self.active_fields().map(StructField::reflected_type)),
+            Some(self.active_types().iter()),
         )
     }