lang/go: relax module consistency checks if vendor/modules.txt is missing

Starting from go1.14, go verifies that vendor/modules.txt matches the
requirements and replacements listed in the main module go.mod file, and it is
a hard failure if vendor/modules.txt is missing.

Relax module consistency checks and switch back to pre go1.14 behaviour if
vendor/modules.txt is missing and GO_NO_VENDOR_CHECKS=1 is set in the
environment regardless of go version requirement in go.mod.

Upstream PR: https://github.com/golang/go/issues/37948

PR: 244783
Reported by: Christopher Hall <hsw@bitmark.com>
Reviewed by: mikael swills yuri
Approved by: jlaffaye (maintainer timeout, 2 weeks)
MFH: 2020Q2
Differential Revision: https://reviews.freebsd.org/D24122