Import symbols in the top-level namespace

Some tools are not smart enough to do proper semantic analysis. That is
the case for `pydoclint`, so it can't figure out that
`_receiver.ReceiverStoppedError` is what we want to document. To make
things simpler we just import the individual symbols to the top-level
namespace.

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

src/frequenz/channels/event.py

@@ -16,10 +16,10 @@
 
 import asyncio as _asyncio
 
-from frequenz.channels import _receiver
+from frequenz.channels._receiver import Receiver, ReceiverStoppedError
 
 
-class Event(_receiver.Receiver[None]):
+class Event(Receiver[None]):
     """A receiver that can be made ready directly.
 
     # Usage
@@ -161,7 +161,7 @@ def consume(self) -> None:
             ReceiverStoppedError: If this receiver is stopped.
         """
         if not self._is_set and self._is_stopped:
-            raise _receiver.ReceiverStoppedError(self)
+            raise ReceiverStoppedError(self)
 
         assert self._is_set, "calls to `consume()` must be follow a call to `ready()`"