Reference kex and mac algo constants from `gitlab-org/golang-crypto`

This is purely a cosmetic change, but it makes code navigation between the projects easier and hopefully help avoid drift in supported algorithms, ciphers, and other protocol-related config.

go.mod has been updated to point to gitlab.com/gitlab-org/golang-crypto@marshall_algo-consts so it will compile, but this should be updated when a release is cut that includes golang-crypto!5 (closed).