Fixes module path finding in the Python code generator and the signatures
of generated servicer methods.

The server_host_override flag's implementation remaining
outstanding means that this only works over insecure
connections for now.

This string comes from an authority field, which is allowed to contain a
':' port (see https://tools.ietf.org/html/rfc3986#section-3.2).

We need to strip it before performing host name verification.

This makes grpc.early_adopter much more independent of RPC
Framework and cleaner at the cost of reexporting most of the
interfaces and writing several delegation classes.