feat(nix/readTree): allow arguments to depend on tree location

This accepts a function for args, and passes it an attrset containing
the locatedAt parameter.

Change-Id: I3c0f8ca00605e02a787fda88f32b06f5ef9998f2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1639
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

nix/readTree/default.nix

@@ -17,9 +17,9 @@ let
     readDir
     substring;
 
-  argsWithPath = parts: args // {
+  argsWithPath = parts:
-    locatedAt = parts;
+    let meta.locatedAt = parts;
-  };
+    in meta // (if isAttrs args then args else args meta);
 
   readDirVisible = path:
     let