Python source reorganization.

src/python/src/grpc/framework/base/packets/interfaces.py

src/python/src/grpc/framework/base/packets/null.py

src/python/src/grpc/framework/base/packets/packets.py

src/python/src/grpc/framework/base/util.py

src/python/src/grpc/framework/common/__init__.py

src/python/src/grpc/framework/common/cardinality.py

src/python/src/grpc/framework/face/__init__.py

src/python/src/grpc/framework/face/_calls.py

src/python/src/grpc/framework/face/_control.py

src/python/src/grpc/framework/face/_service.py

src/python/src/grpc/framework/face/_test_case.py

...framework/face/blocking_invocation_inline_service_test.py

src/python/src/grpc/framework/face/demonstration.py

...k/face/event_invocation_synchronous_event_service_test.py

src/python/src/grpc/framework/face/exceptions.py

...face/future_invocation_asynchronous_event_service_test.py

src/python/src/grpc/framework/face/implementations.py

src/python/src/grpc/framework/face/interfaces.py

src/python/src/grpc/framework/face/testing/__init__.py

src/python/src/grpc/framework/face/testing/base_util.py