[release-branch.go1.16] cmd/go: allow '+' in package import paths in module mode

This change upgrades x/mod to pull in the fix from CL 300152.

Updates #44776.
Fixes #44885.

Change-Id: I273f41df2abfff76d91315b7f19fce851c8770d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/300176


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
(cherry picked from commit d33e2192)
Reviewed-on: https://go-review.googlesource.com/c/go/+/300153