- Add a check to validate DIR_METADATA files.
- Add a check to enforce that OWNERS files contain no metadata if a
  DIR_METADATA file is present in the same directory.

Bug: 1102997
Change-Id: Iedb09941001577dc81b1dda2241481ff667b2568
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2298330
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Nodir Turakulov <nodir@chromium.org>