hack/make/validate-gofmt: switch to goimports

Docker-DCO-1.1-Signed-off-by: Johan Euphrosine <proppy@google.com> (github: proppy)
hack/make/validate-gofmt: add goimports instructions

Docker-DCO-1.1-Signed-off-by: Johan Euphrosine <proppy@google.com> (github: proppy)