This version can load external proto descriptors. No other significant changes.

TBR=tandrii@chromium.org, iannucci@chromium.org
BUG=986972

Change-Id: I26317512ffcee0a50e051032d30ec79bbd805644
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1778707
Commit-Queue: Vadim Shtayura <vadimsh@chromium.org>
Auto-Submit: Vadim Shtayura <vadimsh@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@google.com>