This CL pulls in a fix to golang.org/x/mod/modfile. No change needed
to cmd/go.

Fixes #35737

Change-Id: I7ca1bb46d2923b01587042f0f312d3c3df54c425
Reviewed-on: https://go-review.googlesource.com/c/go/+/208977
Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>