twitter: use import_source and proper merging for tweets from different sources

+ use proper datetime_aware for created_at

my/fbmessenger/common.py

@@ -1,3 +1,5 @@
+from my.core import __NOT_HPI_MODULE__
+
 from datetime import datetime
 from typing import Iterator, Optional, TYPE_CHECKING
 
@@ -35,7 +37,9 @@ class Message(Protocol):
 
 from itertools import chain
 from more_itertools import unique_everseen
-from my.core import Res
+from my.core import warn_if_empty, Res
+
+@warn_if_empty
 def _merge_messages(*sources: Iterator[Res[Message]]) -> Iterator[Res[Message]]:
     # todo might be nice to dump some stats for debugging, e.g. how many were overlapping?
     def key(r: Res[Message]):