Refactor select with new maybe selector

This is a great refactor of the select functionality in the flake class.
This now uses the same parser as the nix code, but runs it in python for
nice stacktraces.

Also we now have a maybe selector which can be used by prepending the
selector with a ?

Tests have been expanded to make sure the code is more stable and easier
to understand

lib/default.nix

@@ -30,6 +30,6 @@ lib.fix (clanLib: {
   # Plain imports.
   values = import ./introspection { inherit lib; };
   jsonschema = import ./jsonschema { inherit lib; };
-  select = import select/default.nix;
+  select = self.inputs.nix-select.lib;
   facts = import ./facts.nix { inherit lib; };
 })