presubmit_support.py should correctly find and use .vpython files.
This will prevent issues like https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/1874458

Bug: 1002153
Change-Id: I677f40b1fd2c436867cb5c6cbdeaff0d16f33a2f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1874892Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>