Add "tools/build"'s "url" and "gitiles" recipe modules into
"depot_tools". "gitiles" had an explicit dependency on "depot_tools", so
this formalizes this dependency.

To support "url", we vendor "requests" from "tools/build" into
"depot_tools" for it.

BUG=None
TEST=expectations

Change-Id: I1ba634ae264b5c9d3adee20db3397c83a6c52c45
Reviewed-on: https://chromium-review.googlesource.com/483316Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Daniel Jacques <dnj@chromium.org>