Audit additional_proto_files

[dbolduc]

LROs have an annotation for their returned metadata type. Proto syntax does not require imports for the proto files that define these metadata types. In C++ we have to include these files.

Our intention for additional_proto_files was to add includes for these extra *.pb.h headers.

Turns out that when we feed them into protoc we treat them the same as any other service's proto file.

google-cloud-cpp/generator/standalone_main.cc

Lines 314 to 317 in a86e887

	args.emplace_back(service.service_proto_path());
	for (auto const& additional_proto_file : service.additional_proto_files()) {
	args.emplace_back(additional_proto_file);
	}

This leads to some weirdness in the generator config. In notebooks and run, we rely on this behavior to generate some clients (and avoid #7997). In containeranalysis, we accidentally generate a client for grafeas which we did not intend to. Oops.

We should sort this out.