In Debian Bullseye dpkg-dep returns empty output if showformat is any virtual source field

[Uladzimir Bely <ubely@ilbers.de>]

Package: dpkg
Version: 1.20.9
Severity: bug

In Debian Bullseye dpkg-deb returns nothing when asked for some 'source:*' 
field, even if .deb file provides the package that is already installed.

At the same time, dpkg-query output is OK.

Logs from Debian Bullseye chroot:

```
(chr-bullseye-amd64)ubely:~$ sudo dpkg -l | grep "ca-certificates"
ii  ca-certificates  20210119  all  Common CA certificates
(chr-bullseye-amd64)ubely:~$ dpkg-deb --show --showformat '${Package}' /var/
cache/apt/archives/ca-certificates_20210119_all.deb; echo
ca-certificates
(chr-bullseye-amd64)ubely:~$ dpkg-deb --show --showformat '${source:Package}' 
/var/cache/apt/archives/ca-certificates_20210119_all.deb; echo

(chr-bullseye-amd64)ubely:~$ dpkg-query --show --showformat='$
{source:Package}' ca-certificates; echo
ca-certificates
(chr-bullseye-amd64)ubely:~$ dpkg-deb --version
Debian 'dpkg-deb' package archive backend version 1.20.9 (amd64).
```

Under Debign Buster everything is OK:

```
(chr-buster-amd64)ubely:~$ sudo dpkg -l | grep "ca-certificates"
ii  ca-certificates  20200601~deb10u2  all  Common CA certificates
(chr-buster-amd64)ubely:~$ dpkg-deb --show --showformat '${Package}' /var/
cache/apt/archives/ca-certificates_20200601~deb10u2_all.deb; echoca-
certificates
(chr-buster-amd64)ubely:~$ dpkg-deb --show --showformat '${source:Package}' /
var/cache/apt/archives/ca-certificates_20200601~deb10u2_all.deb; echo
ca-certificates
(chr-buster-amd64)ubely:~$ dpkg-query --show --showformat='${source:Package}' 
ca-certificates; echo
ca-certificates
(chr-buster-amd64)ubely:~$ dpkg-deb --version
Debian 'dpkg-deb' package archive backend version 1.19.7 (amd64).
```