cmd/go: add 'go mod vendor -o' flag

Adds a new flag to 'go mod vendor' which overrides the default
'vendor' destination directory. This can be helpful for writing the
vendor tree to a temporary location for use by other tools.
The argument can be a relative or an absolute path.
This flag has no other influence on how the command behaves.

Fixes #47327

Change-Id: I4502931127616b181dc90a2066d2fb57bfe48f96
Reviewed-on: https://go-review.googlesource.com/c/go/+/338149


Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Trust: Bryan C. Mills <bcmills@google.com>
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>