This CL picks up
1. DIR_METADATA discovery optimization, based on git-ls-files
2. Mixin support
3. New subcommand "read" that supersedes "export" and "compute"

Last time it was reverted because depot_tools->src got blocked, because
dirmd was used in Swarming task. The latter was resolved in
https://chromium-review.googlesource.com/c/chromium/src/+/2921324

Bug: 1201822, 1179786
Change-Id: I76a580c5d60b506c1f015890fc07817de553975d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2965866
Commit-Queue: Nodir Turakulov <nodir@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>