Support new fully-bundled Git CIPD packages. These are generated by the
"third_party_packages" infra builder, and require no installer.

BUG=chromium:740171
TEST=local

Change-Id: I6126655359ba981eb18ad1e088fe787100719d46
Reviewed-on: https://chromium-review.googlesource.com/562531
Commit-Queue: Daniel Jacques <dnj@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>