feat(tvix/castore/directory/traverse): use castore Paths

This switches from using std::path::Path to using castore paths.

We can drop some error handling in descend_to, as absolute (or redundant)
paths are not representable.

We however now need to convert from a std::path::Path to our
representation, and decide to accept .. canonicalization, as paths in
EvalIO might contain this. Dealing .. to hop into another store path, if
we encounter this, should be dealt with in a previous step.

Change-Id: I5e94693808420c5d56587c68731252b54755bf93
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11575
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: Connor Brewster <cbrewster@hey.com>
Tested-by: BuildkiteCI

tvix/glue/src/tvix_store_io.rs

@@ -305,6 +305,9 @@ impl TvixStoreIO {
         };
 
         // now with the root_node and sub_path, descend to the node requested.
+        // We convert sub_path to the castore model here.
+        let sub_path = tvix_castore::PathBuf::from_host_path(sub_path, true)?;
+
         directoryservice::descend_to(&self.directory_service, root_node, sub_path)
             .await
             .map_err(|e| std::io::Error::new(io::ErrorKind::Other, e))