Backports per-object grpc_init/deinit and separated-file grpc protoc
codegen (#7538, #8246, #8920).

Almost fixes #8753 (which still needs a test).

To support magical internal build processes, the pb2 files need to be
split into pure-proto and gRPC parts. This performs that split and
further fixes bad module names in the test harness that interfered with
the intended test implementation.

An unfortunate side effect, due to limitations of protoc and holdover
behavior we must support in major version 1.x, is that trash files are
generated in split generation. This shouldn't be a problem in normal
protoc plugin use.

Add docstrings mentioning that the Beta API is deprecated post 0.14.x in

- PrintBetaServicer
- PrintBetaStub
- PrintBetaServerFactory
- PrintBetaStubFactory

These changes should have been included in dc8c3239.

(1) Renamed the "beta" module "implementations" because it hasn't been
monolithic since "interfaces" was factored out of it a few changes
back.

(2) Moved ChannelConnectivity from grpc.beta.beta to
grpc.beta.interfaces since it is constants that don't depend on the
beta implementation.

(3) Moved the Server interface definition from grpc.beta.beta to
grpc.beta.interfaces since it is an interface.

(4) Dropped the "create_" prefix from "create_<...>_channel" functions
to better match the other creation functions throughout the codebase.

grpc namespace and not supported by older compilers. Not sure
why I didn't catch this earlier.

Clean up anything that failed

It was awkward for the generated code to call an API that offered
both insecure_server and secure_server as well as insecure_stub and
secure_stub. With this change there is just a single server function
and a single stub function and security is decided based on arguments
passed.

This is part of a change to ease internal usage of GRPC.

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

With this change the Python code generated by the Python code generator
uses the grpc.early_adopter package and not the grpc.framework.face
package.

Fixes module path finding in the Python code generator and the signatures
of generated servicer methods.