fix: make system-lib imports use namespaces

Previously when you accessed stuff on your lib via snowfall-lib's
internal system-lib (e.g. with lib.snowfall.internal.system-lib.my-namespace.my-lib-function)
the lib which is passed to *your* lib used hardcoded the "internal" name

This makes lib functions in your flake's lib break if they use other lib
functions in your flake's lib

snowfall-lib/internal/default.nix

@@ -30,7 +30,7 @@
         namespace = snowfall-config.namespace;
         lib = snowfall-lib.attrs.merge-shallow [
           base-lib
-          {internal = user-lib;}
+          {"${snowfall-config.namespace}" = user-lib;}
         ];
       };
       libs =