added mdns support breaks dpkg.list file parsing

We have several tools that use the dpkg.list file shipped under usr/share/snappy/dpkg.list inside all the core snaps as input for i.e. checking the sources a snap was built from or querying the CVE tracker (review-tools). With the merge of https://github.com/snapcore/core20/commit/d2163f62de8ffc9664a07b008107055a8cd35d06
this file now carries a fake "avahi-daemon" entry with package version "1000" (which is required to make the libnss packages installable as i understand from the PR code) ...

indeed such a version 1000 does not exist anywhere in the ubuntu archive, so any automatic parsing of the file (with package version lookups) fails. The line should be filtered out when dpkg.list gets generated.