[pkg-go] package naming for kingpin.v3

[Martín Ferrari]

I was getting ready to upload this same package, which I did not find
because of the -unstable suffix.

I think this was a bad idea, as now we will need renaming and transition
packages when v3 is finally released... Same issue with the repo name.

Plus, it is not following current practices, like following upstream git
history.

So, unless anybody sees this as a problem, I will take it over with a
package using the v3 name, a repo with that name, etc.

-- 
Martín Ferrari (Tincho)