Changes in the structure of buildGo.external meant that the package
layout for the protobuf library is now slightly different.
`proto` has been amended to work with the new structure. Callers of
buildGo.proto do not need to be updated (i.e. the example still
works).
Moves the Protobuf & gRPC dependencies to a separate file which uses
buildGo.external to build the dependencies.
The versions are pinned at master of 2019-11-26.