flake.nix: reorganize

 - Kittybox's source code is moved to a subfolder
   - This improves build caching by Nix since it doesn't take changes
     to other files into account
 - Package and test definitions were spun into separate files
   - This makes my flake.nix much easier to navigate
   - This also makes it somewhat possible to use without flakes (but
     it is still not easy, so use flakes!)
 - Some attributes were moved in compliance with Nix 2.8's changes to
   flake schema

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