lib: rename helpers.nix -> default.nix

The old `default.nix` is inlined into the "lib" flake-module, which is
the only place it was used.

This allows "helpers" to take its rightful place at the root of `./lib` 👑

flake-modules/lib.nix

@@ -5,7 +5,21 @@
   ...
 }:
 {
+  perSystem =
+    { pkgs, ... }:
+    {
+      _module.args.helpers = import ../lib { inherit lib pkgs; };
+    };
+
+  # TODO: output lib without pkgs at the top-level
   flake.lib = lib.genAttrs config.systems (
-    lib.flip withSystem ({ pkgs, ... }: import ../lib { inherit pkgs lib; })
+    lib.flip withSystem (
+      { pkgs, ... }:
+      {
+        # NOTE: this is the publicly documented flake output we've had for a while
+        check = import ../lib/tests.nix { inherit lib pkgs; };
+        helpers = import ../lib { inherit lib pkgs; };
+      }
+    )
   );
 }