lib/generators: withRecursion: don't break attr-sets with special attrs

Closes #168327

The issue reported there can be demonstrated with the following
expression:

    → nix-instantiate --eval -E "with import ./. {}; pkgs.lib.options.showDefs [ { file = \"foo\"; value = pkgs.rust.packages.stable.buildRustPackages; } ]"
    error: attempt to call something which is not a function but a string

           at /home/ma27/Projects/nixpkgs/lib/trivial.nix:442:35:

              441|   isFunction = f: builtins.isFunction f ||
              442|     (f ? __functor && isFunction (f.__functor f));
                 |                                   ^
              443|

Basically, if a `__functor` is in an attribute-set at depth-limit,
`__functor` will be set to `"<unevaluated>"`. This however breaks
`lib.isFunction` which checks for a `__functor` by invoking `__functor`
with `f` itself.

The same issue - "magic" attributes being shadowed by `withRecursion` -
also applies to others such as
`__pretty`/`__functionArgs`/`__toString`.

Since these attributes have a low-risk of causing a stack overflow
(because these are flat attr-sets or even functions), ignoring them in
`withRecursion` seems like a valid solution.

lib/generators.nix

@@ -251,6 +251,16 @@ rec {
     }:
       assert builtins.isInt depthLimit;
       let
+        specialAttrs = [
+          "__functor"
+          "__functionArgs"
+          "__toString"
+          "__pretty"
+        ];
+        stepIntoAttr = evalNext: name:
+          if builtins.elem name specialAttrs
+            then id
+            else evalNext;
         transform = depth:
           if depthLimit != null && depth > depthLimit then
             if throwOnDepthLimit
@@ -261,7 +271,7 @@ rec {
           let
             evalNext = x: mapAny (depth + 1) (transform (depth + 1) x);
           in
-            if isAttrs v then mapAttrs (const evalNext) v
+            if isAttrs v then mapAttrs (stepIntoAttr evalNext) v
             else if isList v then map evalNext v
             else transform (depth + 1) v;
       in