new Go support function 'buildGoPackage'

This PR is for a new language-specific build funtion for Go.

I have some practial package expressions that use this fuction that I can append to the PR as well.
Also, I have a script that spits out basic expressions for building a Go package using only a Go import line, but I'm going to wait to do some more testing first.

This will take care of https://github.com/NixOS/nixpkgs/issues/3080.