Cache os.listdir calls.
Cache os.listdir calls.

Should especially help on Mac. Mac code path is using os.listdir in
'find_item_native_case' (other platforms doesn't do that).

R=maruel@chromium.org
BUG=

Review URL: https://codereview.appspot.com/157720043

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