This ensures relative file: directives in OWNERS files also work if
the directory of the owners file gets deleted.

Bug: 1015444
Change-Id: I9471a28a7246513120dd3ebb924f6d64eb50c2df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1870249Reviewed-by: Tamer Tas <tmrts@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@google.com>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>