[client] use scandir to calculate directory size
[client] use scandir to calculate directory size

Bug: 985537
Change-Id: I4e79b55d29f72ee72569b86f64a433c2a04b90ac
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/1714687
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py