feat(readTree): Add special here argument.

It's convenient for depending on sub-tree members of the current file as
well as gives access to siblings.

Change-Id: I74234cec6566177d88d3bc8507fa3f6ec789adb8
Reviewed-on: https://cl.snix.dev/c/snix/+/30098
Reviewed-by: adis bladis <adisbladis@gmail.com>
Tested-by: besadii

nix/readTree/tests/default.nix

@@ -134,6 +134,13 @@ let
     (assertEq "default.nix drv is not changed by readTree"
       tree-tl.default-nix.can-be-drv
       (import ./test-tree-traversal/default-nix/can-be-drv/default.nix { }))
+    (assertEq "`here` argument represents the attrset a given file is part of"
+      (builtins.removeAttrs tree-tl.here-arg [ "__readTree" "__readTreeChildren" "subdir" ])
+      {
+        attr1 = "foo";
+        attr2 = "foo";
+        attr3 = "sibl1";
+      })
   ];
 
   # these each call readTree themselves because the throws have to happen inside assertThrows

nix/readTree/tests/test-tree-traversal/here-arg/default.nix

@@ -0,0 +1,6 @@
+{ here, ... }: {
+  attr1 = "foo";
+  attr2 = here.attr1;
+
+  attr3 = here.subdir.sibl2;
+}

nix/readTree/tests/test-tree-traversal/here-arg/subdir/sibl1.nix

@@ -0,0 +1 @@
+{ ... }: "sibl1"

nix/readTree/tests/test-tree-traversal/here-arg/subdir/sibl2.nix

@@ -0,0 +1,2 @@
+{ here, ... }:
+here.sibl1