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

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