feat(tvix/eval): Add a struct implementing NIX_PATH

Add a simple struct implementing both the string parsing and path
resolution rules of Nix's `NIX_PATH` environment variable, for use in
resolving `<...>`-style paths

Change-Id: Ife75f39aa5c12928278d81fe428fbadc98bac5cc
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6917
Autosubmit: grfn <grfn@gws.fyi>
Reviewed-by: tazjin <tazjin@tvl.su>
Reviewed-by: Adam Joseph <adam@westernsemico.com>
Tested-by: BuildkiteCI

tvix/eval/src/lib.rs

@@ -12,6 +12,7 @@ mod value;
 mod vm;
 mod warnings;
 
+mod nix_path;
 #[cfg(test)]
 mod properties;
 #[cfg(test)]