prioritise_build_queue: subpkgs -> main pkgs (MR 2388)

Adjust the dependency resolver, so it uses the main pkgnames in
dependency resolution, instead of subpackages. This fixes for example
that it couldn't find -dev packages even though the binary packages for
those existed.

> systemd: missing dependency linux-pam-dev, trying to queue other packages first
> linux-pam: missing dependency systemd-dev, trying to queue other packages first
> dbus: missing dependency systemd-dev, trying to queue other packages first