migrate to go modules from vndr

Context

Go modules is the standard way to manage dependencies in go https://docs.gitlab.com/ee/development/go_guide/#modules

Summary of Changes

include commit from upstream "replace rsc.io/letsencrypt in favour of golang.org/x/crypto" this upstream commit make the switch more convenient

go.mod and go.sum files added, these files are used by the module system to mange dependencies

The gitlab-ci.yml file is modified to take advantage of caching and to remove now unneeded GOPATH hacking

the vendor/ directory is removed, as it recommend from the GitLab Go Guild linked above

The Makefile exports environment variables which ensure that gomodules is always used in the build process. This ensures that GNC builds complete successfully

Discussion

While the GNC builds succeed it's possible that some of the changes made in gitlab-org/build/CNG!385 (merged) are no longer strictly needed.