Use go mod

Govendor is obsolete, go mod is the new way to manage dependencies in Go.

I used the following script to check that the go.mod file is "right", or at least stable:

sh -c '
set -xe

# simulate situation before go.mod
cd go
git checkout f34e2cd vendor
git reset HEAD
rm go.mod go.sum

# create go.mod
go mod init gitlab.com/gitlab-org/gitlab-shell/go
rm -rf vendor

# force go to download / resolve dependencies
cd ..
bin/compile
support/go-test

# check if go.mod looks different
cd go
go mod tidy
git status
'

I include the information above because this diff is so huge, it's very hard to know what's changing. The script above proves that at least we're doing whatever go mod's auto-govendor-migrator does.