To sanely support concurrency, I need to make `api.step.nest` strictly
scoped; i.e. once you leave the nest context it closes all the steps
within that context.

This changes some wild 'active_step' uses to generate a step for the
module's own purpose.

R=martiniss@chromium.org, nodir@chromium.org, tandrii@chromium.org

Recipe-Nontrivial-Roll: build
Bug: 910369
Change-Id: I19aef31118faf9d3d078a2c5a0ebd48a4bfebd43
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1630044
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>