[release-branch.go1.16] cmd: upgrade golang.org/x/mod to relax import path check

This incorporates CL 298009, which allows leading dots in import path
elements but not module path elements. Also added a test.

Fixes #44647
Updates #34992

Change-Id: I2d5faabd8f7b23a7943d3f3ccb6707ab5dc2ce3c
Reviewed-on: https://go-review.googlesource.com/c/go/+/297530


Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
(cherry picked from commit 97bdac03)
Reviewed-on: https://go-review.googlesource.com/c/go/+/297912


Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>