fix(tvix/castore/import): deal with entry.path() not having a parent

We got away with not properly dealing with this for the archive case,
where everything is contained inside a toplevel dir, but NARs can encode
a single file/symlink.

Properly break if the IngestionEntry path has the ROOT as parent, and
only create filling directories in the other case.

Change-Id: Ib378d0d1040de7c3fe310912a0b0488c55afee83
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11590
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: Connor Brewster <cbrewster@hey.com>

tvix/castore/src/import/mod.rs

@@ -121,15 +121,17 @@ where
             }),
         };
 
-        if entry.path().components().count() == 1 {
+        let parent = entry
-            break node;
+            .path()
-        }
+            .parent()
+            .expect("Tvix bug: got entry with root node");
 
-        // record node in parent directory, creating a new [Directory] if not there yet.
+        if parent == crate::Path::ROOT {
-        directories
+            break node;
-            .entry(entry.path().parent().unwrap().to_owned())
+        } else {
-            .or_default()
+            // record node in parent directory, creating a new [Directory] if not there yet.
-            .add(node);
+            directories.entry(parent.to_owned()).or_default().add(node);
+        }
     };
 
     assert!(