feat(tvix/eval): allow extending builtins outside of tvix_eval

The change allows applications that use tvix_serde for parsing
nix-based configuration to extend the language with domain-specific
set of features.

Change-Id: Ia86612308a167c456ecf03e93fe0fbae55b876a6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8848
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/builtins/impure.rs

@@ -7,6 +7,7 @@ use std::{
 };
 
 use crate::{
+    self as tvix_eval,
     errors::ErrorKind,
     io::FileType,
     value::NixAttrs,