cmd/go: introduce WriteOpts argument for WriteGoMod

cmd/go: introduce WriteOpts argument for WriteGoMod

This CL is a no-op, just adding the new options and plumbing it through.
'go get' will use this option to let commitRequirements know whether
toolchain was mentioned explicitly on the command line.

For #57001.

Change-Id: Iee7145f3335e899704df3e98fb840f1aa4063b0c
Reviewed-on: https://go-review.googlesource.com/c/go/+/499555
Run-TryBot: Russ Cox <rsc@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>