Fix the arch_matches implementation.
Only ever externally block build jobs
contrib/tanglu: Use dsc filename from rapidumo PackageInfo object instead of parsing Sources.gz a second time
trivial: Remove hook-removals from Tanglu buildd instructions
create_job(): Do not create build jobs for arches with pre-existing binaries.

When not creating a build Job, create a Binary instead, so binary jobs can be started immediately.
This requires Binary.build_job to nullable, as there now might be binaries that wasn't built by a debile Job.
As a result Binary.arch must be a column instead of a hybrid_property pointing to the arch of the build job (which may not exist).
contrib/tanglu: Update Tanglu glue script
Pep8ify update-jobs.py