Make custom, functions and types proper modules

This allows "from telethon.types import Message" to work.
Closes #3929.

Not entirely sure how it used to work before, perhaps
it got changed at some point but this should revert previous
behaviour.

telethon/__init__.py

@@ -1,9 +1,8 @@
 from .client.telegramclient import TelegramClient
 from .network import connection
-from .tl import types, functions, custom
 from .tl.custom import Button
 from .tl import patched as _  # import for its side-effects
-from . import version, events, utils, errors
+from . import version, events, utils, errors, types, functions, custom
 
 __version__ = version.__version__
 

telethon/custom.py

@@ -0,0 +1 @@
+from .tl.custom import *

telethon/functions.py

@@ -0,0 +1 @@
+from .tl.functions import *

telethon/types.py

@@ -0,0 +1 @@
+from .tl.types import *