Debian loader: TypeError: __init__() missing 1 required positional argument: 'md5sum'

SWH-LOADER-CORE-XN when loading http://deb.debian.org/debian//pool/main/h/horizon/horizon_14.0.2.orig.tar.gz

Looks like we need to make the md5sum argument of DebianFileMetadata optional

---

Migrated from T3547 (view on Phabricator)