rework building protos with protoc changes

[mjheilmann]

The protoc changes when updating protobuf alters a few behaviors. Specifically it now tries to respect the file input path when generating the file output path. But it also respects the package name within the proto file, so both the package path and the input path are used for the output path, leading to things like grpc/v1/grpc/v1/reflection.pb.ex, which is not desired.

In this PR:

• update protobuf and use the newer protoc
• relocate test structures so their inputs are package-pathed
• update the proto generation to read the input package path and use if as an I argument on protoc, so the input path can be ignored
• relocate the pb.ex files to be at their package paths as generated, so they can be reliably regenerated with the protoc changes
• rebuild the pb.ex files using protobuf 0.17