auto-decruft: Ignore Build-Depends from crufty source packages

The rm and cruft-report commands use the check_reverse_depends from
utils, which was modified to not consider crufty source packages'
Build-Depends in d7b8a9f2 (#792578),
whereas auto-decruft uses ReverseDependencyChecker's implementation that
was never modified and thus fails to pick up some cruft that
cruft-report says has no dependency problem.