Bug#913925: debsums incorrectly reports package as not installed

[bss at iguanasuicide.net]

Package: debsums
Version: 2.2.2
Severity: normal

Dear Maintainer,

See this zsh session:
---8<---
% sudo debsums -s spamassassin
debsums: package spamassassin is not installed
% apt-cache policy spamassassin
spamassassin:
  Installed: 3.4.2-1~deb9u1
  Candidate: 3.4.2-1~deb9u1
  Version table:
     3.4.2-1 700
        700 http://mirrors.gandi.net/debian buster/main amd64 Packages
        500 http://mirrors.gandi.net/debian sid/main amd64 Packages
 *** 3.4.2-1~deb9u1 900
        900 http://mirrors.gandi.net/debian stretch/main amd64 Packages
        100 /var/lib/dpkg/status
--->8---

Clearly, either apt-cache or debsums is wrong, and since spamassassin just
scanned the last mail traveling through this server, I'm betting debsums is
wrong.


-- System Information:
Debian Release: 9.6
  APT prefers stable-updates
  APT policy: (900, 'stable-updates'), (900, 'stable'), (850, 'proposed-updates'), (700, 'testing'), (500, 'unstable'), (300, 'experimental')
Architecture: amd64 (x86_64)

Kernel: Linux 4.9.0-7-amd64 (SMP w/1 CPU core)
Locale: LANG=en_US.utf8, LC_CTYPE=en_US.utf8 (charmap=UTF-8), LANGUAGE=en_US (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: sysvinit (via /sbin/init)

Versions of packages debsums depends on:
ii  dpkg                  1.18.25
ii  libdpkg-perl          1.18.25
ii  libfile-fnmatch-perl  0.02-2+b3
ii  perl                  5.24.1-3+deb9u4
ii  ucf                   3.0036

debsums recommends no packages.

debsums suggests no packages.

-- debconf information:
  debsums/apt-autogen: true