lib: Add recurseIntoAttrs

This makes the function available without having to evaluate the
Nixpkgs fix-point, making it available in a more natural way for
code that deals with multiple Nixpkgs invocations.

Its definition is coupled to Nix rather than Nixpkgs, so it will
feel right at home in lib.

lib/attrsets.nix

@@ -473,6 +473,12 @@ rec {
   /* Pick the outputs of packages to place in buildInputs */
   chooseDevOutputs = drvs: builtins.map getDev drvs;
 
+  /* Make various Nix tools consider the contents of the resulting
+     attribute set when looking for what to build, find, etc.
+   */
+  recurseIntoAttrs =
+    attrs: attrs // { recurseForDerivations = true; };
+
   /*** deprecated stuff ***/
 
   zipWithNames = zipAttrsWithNames;

lib/default.nix

@@ -70,7 +70,8 @@ let
       genAttrs isDerivation toDerivation optionalAttrs
       zipAttrsWithNames zipAttrsWith zipAttrs recursiveUpdateUntil
       recursiveUpdate matchAttrs overrideExisting getOutput getBin
-      getLib getDev chooseDevOutputs zipWithNames zip;
+      getLib getDev chooseDevOutputs zipWithNames zip
+      recurseIntoAttrs;
     inherit (lists) singleton forEach foldr fold foldl foldl' imap0 imap1
       concatMap flatten remove findSingle findFirst any all count
       optional optionals toList range partition zipListsWith zipLists