style(flake)!: manually import lib flake module

- Also make `lib` an `internal` option
- Unify let blocks and move them under `imports`
  (makes it clear where `config.lib` comes from)
- Fix a minor typo

modules/flake/lib/default.nix

@@ -6,9 +6,12 @@
   ];
 
   options = let
-    inherit (lib) types;
+    inherit (lib)
+      types
+      ;
   in {
     lib = lib.mkOption {
+      internal = true;
       type = types.unspecified;
     };
   };