Avoid relative imports in grpc code

[nipunn1313]

Fixes #277

[nipunn1313]

Would appreciate a review from either @MHDante or @Evgenus as well as they've contributed majorly to the GRPC stubs here.

My rationale is that this matches more closely to how the _grpc_pb2.py work, and obviates need for __init__.py files (at least matches more closely w/ how the .py works.

[Evgenus]

Looks good but what if generated files are not in root package?

[nipunn1313]

I believe that you need to add your generated files directory to your PYTHONPATH or sys.path for things to work (see protocolbuffers/protobuf#881). I think it's just a shortcoming of protobuf. Looks like the task was wont-fixed in 2017. I don't think that use case is well supported unfortunately.

mypy will find imports via this mechanism https://mypy.readthedocs.io/en/stable/running_mypy.html#finding-imports

For example, in our testsuite, the top of dummy_pb2_grpc.py looks like this

# Generated by the gRPC Python protocol compiler plugin. DO NOT EDIT!
"""Client and server classes corresponding to protobuf-defined services."""
import grpc

from testproto.grpc import dummy_pb2 as testproto_dot_grpc_dot_dummy__pb2

^ not using relative import.

With this diff, the dummy_pb2_grpc.pyi will match that - and thus work more similarly in cases like the one in #277