cmd/go: support the -overlay flag for go mod commands
Move the declaration of the -overlay flag to base.AddModCommonFlags,
where other flags that are needed for go mod commands and for builds
are declared. The flag's already initialized in modload.Init so
there's no additional work needed to be done to support it in the go
mod commands.
For #39958
Change-Id: I70725d620cc69cb820f6ed923d626f4fe041b1c5
Reviewed-on: https://go-review.googlesource.com/c/go/+/272126
Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Bryan C. Mills <bcmills@google.com> Reviewed-by: Jay Conrod <jayconrod@google.com>