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

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

Refactors early_adopter such that interface-specific code is located in
the framework module as a submodule `alpha`.

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.