Rename module to entire.io/git-sync

Mechanical rename of the Go module path from github.com/entirehq/git-sync to entire.io/git-sync. The git remote stays at github.com/entirehq/git-sync; this is a vanity import path change only.

Do not merge until entirehq/entire.io#1730 ships to production — that PR adds the go-import meta tag at https://entire.io/git-sync, without which 'go get entire.io/git-sync' fails for downstream consumers.