Re-export catalogs

[notgne2]

This seems to improve usability when trying to use the nixpkgs catalog in isolated cases, an example of this being our example usage of mkEnv, where we have to add this passthru https://github.com/flox-examples/floxEnv/blob/31d0ce3e7d2c5f2c67903546940df7ae40ed3159/flake.nix#L9-L11 so that it can be used here https://github.com/flox-examples/floxEnv/blob/31d0ce3e7d2c5f2c67903546940df7ae40ed3159/pkgs/default.nix#L6.

With this change we should be able to get rid of the passthru and write with inputs.floxpkgs.catalogs.nixpkgs.evalCatalog;, and without this change or the passthru we would need to change it to with inputs.floxpkgs.inputs.nixpkgs.${system}.evalCatalog; (and acquire system from somewhere too).

I've spoken with @ysndr about the possibility of de-system-ing inputs of inputs, but it sounds like that would be difficult, and semantically it is weird to "abuse" floxpkgs as a way to import other things from its inputs if it isn't re-exporting them.

Not sure if this is the best format to do this in, maybe we could have passthru.catalogs and passthru.evalCatalogs separately? I actually don't know enough about them yet to know what the difference is.