Build-Depends: fix handling of multiple blockers
When a source item has multiple build-depends blockers on an architecture, only the last one was preserved. Fix this by keeping all blockers.
Directly add BinaryPackageId to the set, because BinaryPackage is not hashable, and we only need the BinaryPackageId later on anyway.
Signed-off-by: Ivo De Decker ivodd@debian.org