fix failure of PPM when instaling a package with a dependency from security

[dimkr]

PPM tries to use one mirror for all packages, but that won't work in some case.

For example, installation of qemu-system-gui fails, because some of its dependencies come from main, but some come from security (because they have a security update).

The fix is to select the mirror to use for each file, instead of using the same one for all of them:

~$ tail -n 2 /tmp/petget_proc/download_urls  
https://ftp.de.debian.org/debian/pool/main/s/seabios/seabios_1.14.0-2_all.deb
http://deb.debian.org/debian-security/pool/updates/main/q/qemu/qemu-system-gui_5.2+dfsg-11+deb11u1_amd64.deb