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].
Signed-off-by: Enji Cooper <ngie@FreeBSD.org>