Make repository format more dynamic

[soredake]

Originally posted here: https://discourse.ubuntu.com/t/what-do-you-want-to-see-in-apt/16588/21?u=soredake

I want apt to have a way to define repos like in dnf, current apt format forces you to specify distro name, focal for example, deb http://ppa.launchpad.net/fish-shell/release-3/ubuntu focal main, and after every distro upgrade you need to re-enable repos and change distro codename in apt.sources. dnf repos works differently metalink=https://mirrors.rpmfusion.org/metalink?repo=free-fedora-source-$releasever&arch=$basearch, distro name is replaced with env variable, which is nice, and after distro update repos does not get disabled and don't need to change distro codename, as it's dynamic, which is awesome.