general: extract the hack to warn of legacy imports and fallback to core/legacy.py

use it both in my.fbmessenger and my.reddit

if in the future any new modules need to be switched to namespace package structure with all.py it should make it easy to do

related:
- https://github.com/karlicoss/HPI/issues/12
- https://github.com/karlicoss/HPI/issues/89
- https://github.com/karlicoss/HPI/issues/102

misc/check_legacy_init_py.py

@@ -12,7 +12,7 @@ import logzero # type: ignore[import]
 logger = logzero.logger
 
 
-MSG = 'DEPRECATED! Instead of my.fbmessengerexport'
+MSG = 'importing my.fbmessenger is DEPRECATED'
 
 def expect(*cmd: str, should_warn: bool=True) -> None:
     res = run(cmd, stderr=PIPE)