Mk/Uses/go.mk: Allow naming output executable in GO_TARGET

• Allow GO_TARGET to be specified as a tuple in the form package:output
• Rework build/install targets

PR: 240535
Submitted by: Dmitri Goutnik <dg@syrec.org>
Exp-run by: antoine
Differential Revision: https://reviews.freebsd.org/D21562