Bug#892174: apt-get source -t fails if target release doesn't have deb sources.list entry

[Harald Sitter <sitter@kde.org>]

Package: apt
Version: 1.2.25

simple example in a docker container `docker run -it --rm ubuntu:bionic bash`

```
echo 'deb-src http://archive.ubuntu.com/ubuntu xenial main' >
/etc/apt/sources.list.d/xenial.list
apt update
apt-get source -t xenial apt
```

Will fail because the xenial release has no `deb ...` sources.list
entry and the pkgPolicy code checking -t is valid only iterates the
package cache it seems. Adding a suitable deb entry makes this work.

```
echo 'deb http://archive.ubuntu.com/ubuntu xenial main' >>
/etc/apt/sources.list.d/xenial.list
apt update
apt-get source -t xenial apt
```

pkgPolicy should find the target release even when only the 'deb-src'
entry is there.