Mk/Uses/go.mk: Remove support for installing Go libs and sources

All ports that depend on it have been removed in r497504.

• Restrict build to only main packages by adding -buildmode=exe to GO_BUILDFLAGS
• Add initial support for building in module-aware mode

PR: 236509
Submitted by: Dmitri Goutnik <dg@syrec.org>
Reviewed by: 0mp, jlaffaye, tobik
Approved by: jlaffaye (maintainer)
Differential Revision: https://reviews.freebsd.org/D19570