[release-branch.go1.16] cmd: upgrade golang.org/x/mod to fix go.mod parser

modfile.Parse passed an empty string to the VersionFixer for the
module path. This caused errors for v2+ versions.

For #44496

Change-Id: I13b86b6ecf6815c4bc9a96ec0668284c9228c205
Reviewed-on: https://go-review.googlesource.com/c/go/+/296131


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 bcac57f8)
Reviewed-on: https://go-review.googlesource.com/c/go/+/297990


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