Migrate recipes away from buildbucket.build.infra.swarming to backend

Bug: https://issues.chromium.org/issues/326592434
Change-Id: I00da7e517bcd272685095e5e68db6458a7a985c2
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/355720
Reviewed-by: Jonas Termansen <sortie@google.com>
Commit-Queue: William Hesse <whesse@google.com>
7 files changed