refactor(tvix/store): import_path → import_path_as_nar_ca

Add multiple additional helpers such as:

- `path_to_name`: derive the basename of a given path
- `derive_nar_ca_path_info`: derive the `PathInfo` for a content
  addressed NAR

which isolates further the tree walking feature and the ingestion feature.

Additionally, we don't `expect` anymore and propagate properly ingestion errors up.

Change-Id: I60edb5b633911c58ade7e19f5002e6f75f90e262
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10574
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Autosubmit: raitobezarius <tvl@lahfa.xyz>

tvix/glue/src/tvix_store_io.rs

@@ -274,7 +274,7 @@ impl EvalIO for TvixStoreIO {
     #[instrument(skip(self), ret, err)]
     fn import_path(&self, path: &Path) -> io::Result<PathBuf> {
         let output_path = self.tokio_handle.block_on(async {
-            tvix_store::utils::import_path(
+            tvix_store::import::import_path_as_nar_ca(
                 path,
                 &self.blob_service,
                 &self.directory_service,