This is a partial reland of commit
e8ef6259, but without actually upgrading
gsutil.

This refactoring allows easier upgrade of gsutil.

R=dpranke@google.com

Bug: 1249003
Change-Id: I63433b411e640a9fdb455ce25a51d910be0818a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3166676Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>