Fix publicised groups API (singular) over federation

which was missing its fed client API, since there is no other API
it might as well reuse the bulk one and unwrap it

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

synapse/handlers/groups_local.py

@@ -383,11 +383,11 @@ class GroupsLocalHandler(object):
 
             defer.returnValue({"groups": result})
         else:
-            result = yield self.transport_client.get_publicised_groups_for_user(
-                get_domain_from_id(user_id), user_id
-            )
+            result = yield self.transport_client.bulk_get_publicised_groups(
+                get_domain_from_id(user_id), [user_id],
+            )["users"][user_id]
             # TODO: Verify attestations
-            defer.returnValue(result)
+            defer.returnValue({"groups": result})
 
     @defer.inlineCallbacks
     def bulk_get_publicised_groups(self, user_ids, proxy=True):