This CL adds the ignore directive which enables users to tell the Go
Command to skip traversing into a given directory.
This behaves similar to how '_' or 'testdata' are currently treated.
This mainly has benefits for go list and go mod tidy.
This does not affect what is packed into a module.
Fixes: #42965
Change-Id: I232e27c1a065bb6eb2d210dbddad0208426a1fdd
Reviewed-on: https://go-review.googlesource.com/c/go/+/643355
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Michael Matloob <matloob@golang.org> Reviewed-by: Michael Matloob <matloob@google.com>