[dev.cmdgo] cmd: pull in x/mod on the dev.cmdgo branch

This change changes the requirement on golang.org/x/mod to require the
top of the dev.cmdgo branch of that repository. This allows the code
on this branch to use the changes on the branch on that repo for parsing
go.work files. It also vendors in those changes using go mod vendor.

Generated using:
  go get -d golang.org/x/mod@dev.cmdgo
  go mod tidy
  go mod vendor

For #45713

Change-Id: I7a20835937c13f90d17c0f39d96b435daec89fba
Reviewed-on: https://go-review.googlesource.com/c/go/+/334933


Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>