fix(readTree): Apply .skip-subtree to Nix-file children as well

This behaviour was previously confusing, since readTree's data
structure treats children from Nix files and directories as identical
but only one of them would be affected by .skip-subtree

The "subtree" to be skipped here refers to all children of the
structure.

Change-Id: Idf596c9823f09cc2acf49523916bde4b801b8519

nix/readTree/default.nix

@@ -100,7 +100,8 @@ let
       }) (filter filterDir (attrNames dir));
 
       # Import Nix files
-      nixFiles = filter (f: f != null) (map nixFileName (attrNames dir));
+      nixFiles = if hasAttr ".skip-subtree" dir then []
+        else filter (f: f != null) (map nixFileName (attrNames dir));
       nixChildren = map (c: let
         p = joinChild (c + ".nix");
         childParts = parts ++ [ c ];