Allow archive_name to be debian-security

This fixes this failure:

fetch https://qa.debian.org/madison.php?package=qemu&yaml=on&s=buster-security
remote version: 1:3.1+dfsg-8+deb10u4
update to 1:3.1+dfsg-8+deb10u4
fetch https://snapshot.debian.org/mr/package/qemu/1:3.1+dfsg-8+deb10u4/srcfiles?fileinfo=1
Traceback (most recent call last):
  File "/usr/bin/apertis-pkg-pull-updates", line 242, in <module>
    main()
  File "/usr/bin/apertis-pkg-pull-updates", line 232, in main
    dsc = mirror.rstrip('/') + '/' + get_remote_dsc_path(package_name, remote_version)
  File "/usr/bin/apertis-pkg-pull-updates", line 191, in get_remote_dsc_path
    raise KeyError((package_name, version))
KeyError: ('qemu', Version('1:3.1+dfsg-8+deb10u4'))