This commit adds the staging repos to the GOPATH in
hack/verify-godeps.sh. This allows vendored librarys
to depend on staging repos and not break verify-godeps.
This also adds `hack/godep-restore.sh`, which acts like
`godep restore`, but sets the GOPATH appropriate as well.
This should fix#36110. The current flake rate is 0.5%, so with the
unrealistic assumption of uncorrelated flakes, a single retry will bring
it down to <0.01% flake rate.
This allows us to start building real dependencies into Makefile.
Leave old hack/* scripts in place but advise to use 'make'. There are a few
rules that call things like 'go run' or 'build/*' that I left as-is for now.