When hooks multiple cipd packages under same directory, gclient would
pop up error that same directory name appears multiple times in deps
section. This CL overrides verify_validity for CipdDependency to
support multiple packages hooked in same directory.

Bug:812386
Change-Id: Ia4f1fe0e3c8481c9b06c1d22b6e98d98e1e4c309
Reviewed-on: https://chromium-review.googlesource.com/920686
Commit-Queue: Shenghua Zhang <shenghuazhang@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>