courier-mta is broken: /usr/lib/courier/courier/courierfilter missing from package

The courier-mta package in artful and bionic is broken; it ships /lib/systemd/system/courierfilter.service, which points to /usr/sbin/courierfilter, which references /usr/lib/courier/courier/courierfilter, but this last file is missing from the package (and from the archive).

This problem was detected in the courier-filter-perl autopkgtest, which failed because the courierfilter service doesn't start:

Setting up courier-mta (0.78.0-2) ...
Job for courierfilter.service failed because the control process exited with err
or code.
See "systemctl status courierfilter.service" and "journalctl -xe" for details.
invoke-rc.d: initscript courierfilter, action "start" failed.
ESC[0;1;31m●ESC[0m courierfilter.service - Courier Filter Daemon
   Loaded: loaded (/lib/systemd/system/courierfilter.service; disabled; vendor p
reset: enabled)
   Active: ESC[0;1;31mfailedESC[0m (Result: exit-code) since Tue 2017-10-31 18:2
5:07 UTC; 17ms ago
  Process: 4043 ExecStart=/usr/sbin/courierfilter start ESC[0;1;31m(code=exited,
 status=127)ESC[0m

Oct 31 18:25:07 autopkgtest systemd[1]: Starting Courier Filter Daemon...
Oct 31 18:25:07 autopkgtest courierfilter[4043]: /bin/sh: 1: /usr/lib/courier/courier/courierfilter: not found
Oct 31 18:25:07 autopkgtest systemd[1]: ESC[0;1;39mcourierfilter.service: Control process exited, code=exited status=127ESC[0m

https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac/autopkgtest-bionic/bionic/amd64/c/courier-filter-perl/20171031_182654_c5326@/log.gz

Unfortunately, this somehow did not stop the courier package from migrating to artful, even though a regressing autopkgtest in a reverse-dependency is supposed to always be considered a blocker.