Mk/Uses/go.mk: support v2 and higher proxy URLs
golang projects with versions greater than 2.x have different URLs
presented for them via ${GOPROXY} [1]. In particular, the major
version is added to the URL and to the extracted GOPROXY provided
DISTFILE(s).
This change adjusts MASTER_SITES and WRKSRC with USES+= go to
support the "Major Version Suffixes" documented in the go module
docs [1].
Reviewed by: adamw, arrowd
Approved by: adamw (go)
Differential Revision: https://reviews.freebsd.org/D49723