Add support for the same username across multiple targets

[jameslounds]

Hopefully fixes #124
In the v3 migration docs, it says that a file containing home manager configuration at homes/<target>/ <username>/default.nix will make the corresponding homeConfiguration available as outputs.homeConfigurations."<username>@<target>". In reality, it is available at outputs.homeConfigurations.<username>. It seems that x86_64-linux has precedence over aarch64-darwin (edit: I think because of the order of variables in flake-utils's defaultSystems ), so when I have both homes/aarch64-darwin/james/default.nix and homes/x86_64-linux/james/default.nix, only the x86_64-linux configuration actually makes it to the outputs (and importantly, darwinConfigurations.myMachine.home-manager.users).

This PR does 3 things:

• Makes homeConfigurations work like the migration guide says it should (for the systems in outputs.pkgs - only systems ending in -linux or -darwin can make usable homeConfigurations, but other systems will still add to homeConfigurations. By using attrNames pkgs, we only make packages.<system>.homeConfigurations for systems which can make usable homeConfigurations.

• Uses <username>@<target> for homes defined at ./homes/<target>/<username>/default.nix when checking which homes to apply to darwinConfigurations and nixosConfigurations.

• Adds packages.<target>.homeConfigurations as an output of mkFlake so that home-manager switch --flake . (or ".#<username>") works. In order to do this, we reconstruct homeConfigurations for each defaultSystem, including only the homes for that system. We then rename any homes whose name ends with @<system>, and remove that @<system> suffix. This ensures that a home declared by homes/<system>/<username>/default.nix can be seen by home-manager at packages.<system>.homeConfigurations.<username>

I'm not the happiest about the changes I've made to snowfall-lib/flake/default.nix - I'm sure there's a much nicer way of doing this, but my nix skills aren't there yet.