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

Change names to StreamedUnary, ServerUnaryStreamer, etc. Use a templated method handler since most code shared between the new StreamedUnary and the existing BidiStreaming. Eliminate the separate enum case for streamed unary. Return a status failure if a StreamedUnary method handler doesn't actually do a write (since that is
violating the appearance of unary-ness)

in all cases

relevant methods get treated this way.

ServerReaderWriterInterface so that some functions can be made
to accept either.

using 'using'. Removed unnecessary grpc::string. fixed indentation.

Modified objc plugin to add two types of import statements for well
known protos.
Deleted unnecessary empty.proto as we'll be using wellknown empty.proto
Modified test to use the well known empty.proto.