sys takes essentially no time to import (I think possibly because
it's a built-in?), and importing it as-needed is fragile. Make
it a top-level import to gclient_paths to prevent future breakages.

BUG=939959

Change-Id: I1ebe1260afd927355d668acf3cab5067ff0a2e03
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1515669Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>