Python source reorganization.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

...n/src/grpc/framework/base/packets/implementations_test.py

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