With the latest SDK release, it's version and the version of the supported protocol is now out of sync: the latest SDK is v3, while the latest protocol is v2.

We should split the protocol package into a separate repository. It
will also help to address bblfsh/bblfshd#281, since we will have a Go protocol package version set explicitly in the go.mod file.

Also, may make sense to do the same for the internal driver protocol package.