Fix the Golang import path

Prior to this change, we used to import the path of the `go` directory.
This however, runs into the `go` keyword. So what we do, is alias the
package we import. This change moves it to gitalypb, inspired by the
Uber Prototool styleguide: https://github.com/uber/prototool/blob/dev/etc/style/uber/uber.proto#L38
3 jobs from zj-fix-proto-import-path in 3 minutes and 20 seconds
Status Job ID Name Coverage
  Test
passed #90622024
build

00:02:43

passed #90624325
check-grpc-proto-clients

00:00:36

failed #90622025
check-grpc-proto-clients

00:02:46