gclient previously used a CIPD root per DEPS file. This didn't work
in cases in which a DEPS file wanted to specify a CIPD package outside
of its directory hierarchy, though, as is the case with buildspecs.

Bug: 755920
Change-Id: I0d6c3db567f17f9171c0feaaf9ed6bc64db22757
Reviewed-on: https://chromium-review.googlesource.com/955933
Commit-Queue: John Budorick <jbudorick@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>
Reviewed-by: Michael Moss <mmoss@chromium.org>