walk: move NewListRHelper into list.Helper to avoid circular dependency

It turns out that the list helpers were at the wrong level and needed
to be pushed down into the fs/list for future work.

backend/b2/b2.go

@@ -31,8 +31,8 @@ import (
 	"github.com/rclone/rclone/fs/fserrors"
 	"github.com/rclone/rclone/fs/fshttp"
 	"github.com/rclone/rclone/fs/hash"
+	"github.com/rclone/rclone/fs/list"
 	"github.com/rclone/rclone/fs/operations"
-	"github.com/rclone/rclone/fs/walk"
 	"github.com/rclone/rclone/lib/bucket"
 	"github.com/rclone/rclone/lib/encoder"
 	"github.com/rclone/rclone/lib/multipart"
@@ -918,7 +918,7 @@ func (f *Fs) List(ctx context.Context, dir string) (entries fs.DirEntries, err e
 // of listing recursively that doing a directory traversal.
 func (f *Fs) ListR(ctx context.Context, dir string, callback fs.ListRCallback) (err error) {
 	bucket, directory := f.split(dir)
-	list := walk.NewListRHelper(callback)
+	list := list.NewHelper(callback)
 	listR := func(bucket, directory, prefix string, addBucket bool) error {
 		last := ""
 		return f.list(ctx, bucket, directory, prefix, addBucket, true, 0, f.opt.Versions, false, func(remote string, object *api.File, isDirectory bool) error {