Setuptools was updated and our hacky namespace-package-chickens came
back to roost. This removes the unsupported namespace package hacks.

Fixes #8334.

-Rename namespace to grpc_health->grpc to match spec
-Proper use of NOT_FOUND status code
-Improve testing
-Add source distribution to artifact build

This reorganizes the Python code, scraps the current testing
infrastructure, and implements a simple test discovery and run script
based on the standard Python unittest library so we can trust that our
tests are running.

This is in preparation of moving all tests into a separate package to
enable ease of coverage checking and testing.

Fixes #1591.

Also fixes a raised-exceptions-during-tests-causes-hanging problem.

Also adds the commands to the grpc_docker shell library script to
support running in docker.

Also refactor the interop package to perform client-server interop
testing in a unit test.

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.