refactor(3p/nix/libexpr): Use absl::btree_map for AttrSets

This is the first step towards replacing the implementation of
attribute sets with an absl::btree_map.

Currently many access are done using array offsets and pointer
arithmetic, so this change is currently causing Nix to fail in various
ways.

third_party/nix/meson.build

@@ -354,6 +354,7 @@ absl_deps = [
   absl.dependency('algorithm_container'),
   absl.dependency('base'),
   absl.dependency('bits'),
+  absl.dependency('btree'),
   absl.dependency('city'),
   absl.dependency('config'),
   absl.dependency('container_common'),