refactor(readTree): Move 'drvTargets' into readTree

This function is also generally useful for readTree consumers that
have the concept of subtargets.

Change-Id: Ic7fc03380dec6953fb288763a28e50ab3624d233

nix/utils/default.nix

@@ -1,22 +1,6 @@
 { depot, lib, ... }:
 
 let
-
-  /* Takes an attribute set and adds a meta.targets
-     attribute to it which contains all direct children
-     of the attribute set which are derivations.
-
-     Type: attrs -> attrs
-  */
-  drvTargets = attrs:
-    attrs // {
-      meta = {
-        targets = builtins.filter
-          (x: lib.isDerivation attrs."${x}")
-          (builtins.attrNames attrs);
-      } // (attrs.meta or {});
-    };
-
   /* Get the basename of a store path without
      the leading hash.
 
@@ -186,7 +170,6 @@ let
 
 in {
   inherit
-    drvTargets
     storePathName
     pathType
     isDirectory