mkPackage: Expose fixpoint root as .internals

Giving no access at all would be really annoying for debugging.

Unlike `mkDerivation`, which makes no distinction between intentional
and internal package attributes, this gives a signal that when you're
using it, you are more likely to have to change your code at some point.
(besides keeping the public package attrs _relevant_)

pkgs/build-support/package/make-package.nix

@@ -61,6 +61,8 @@ let
         deps = old.deps // f old.deps;
       });
 
+      internals = this;
+
       # FIXME: Use `toExtension f`?
       # TODO: Support legacy attrs like passthru?
       overrideAttrs = f: