feat(nix/readTree): record list of children added by readTree

This change adds a new attribute to readTree nodes, `__readTreeChildren`
which is a list of attribute names added to this node by readTree.

This is then used by `gather` for `ci.targets` to avoid evaluating
attributes unnecessarily. Especially since Nix is not as lazy as we'd
like when determining types (i. e. child ? __readTree needs to force
`child` even when it's not an attribute set), evaluating attributes
unnecessarily is sometimes problematic.

Change-Id: I0a98691d41f987e23ee7e9ba21fbe465da5fe402

default.nix

@@ -79,13 +79,14 @@ let
       # Include the node itself if it is eligible.
       (if eligible node then [ node ] else [])
       # Include eligible children of the node
-      ++ concatMap gather (attrValues node)
+      ++ concatMap gather (map (attr: node."${attr}") node.__readTreeChildren)
       # Include specified sub-targets of the node
       ++ filter eligible (map
            (k: (node."${k}" or {}) // {
              # Keep the same tree location, but explicitly mark this
              # node as a subtarget.
              __readTree = node.__readTree;
+             __readTreeChildren = [];
              __subtarget = k;
            })
            (node.meta.targets or []))