Rename go package to gitalypb
Prior to this change, we used to import the path of the go
directory.
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
This will fix goimports, for example:
Edited by Zeger-Jan van de Weg