Support for GitHub/GitLab PR URLs about cabal2nix

[sternenseemann]

I was aiming to revisit the fetching code soon in order to find a cleaner solution for #585, so it may not make sense yet to add a new feature in this area at the moment. I.e. it probably makes sense to get a handle on the complexity first to be free to contribute to it later.

In principle I'm not opposed to the change, especially if we can delegate this to a tool in the end. Currently cabal2nix is very stupid and just throws whatever it is getting at a bunch of tools after it has determined that it's not a hackage URL. I'd like to keep it that way, so we don't need to maintain a ton of integrations in cabal2nix or parse URLs etc.