deb: classify the staged listing through the context

The fallback listing of find_package_directory called Path::is_dir
on entries returned by list_files — a host-side stat. For an unshare
context (every local build) those paths are rooted inside the chroot
and do not exist at the same host path, so every entry came out a
non-directory, the 'Found directories' list silently stayed empty
and the failure degraded to the list-less 'Could not find package
directory' variant, hiding the actual layout (seen building bc from
an ubuntu/devel checkout whose staged tree was named 'tree').

Classify entries through the new ContextDriver::is_dir, and log
every entry instead of only those matching the broken host stat.

src/deb/mod.rs

@@ -413,11 +413,13 @@ pub(crate) fn find_package_directory(
         let entries = ctx.list_files(package_parent)?;
         let mut found_dirs = Vec::new();
         for entry in entries {
-            if entry.is_dir() {
-                if let Some(file_name) = entry.file_name() {
-                    found_dirs.push(file_name.to_string_lossy().into_owned());
-                }
-                log::debug!("  - {}", entry.display());
+            // list_files yields context-relative paths (e.g. rooted inside
+            // the chroot for an unshare context): classify through the
+            // context, a host-side stat would miss every entry.
+            let is_dir = ctx.is_dir(&entry)?;
+            log::debug!("  - {}", entry.display());
+            if is_dir && let Some(file_name) = entry.file_name() {
+                found_dirs.push(file_name.to_string_lossy().into_owned());
             }
         }
 
@@ -501,6 +503,51 @@ mod tests {
         }
     }
 
+    /// An unshare context mapped over `chroot_root`, parented on a local
+    /// context like the ephemeral build contexts are: exists/list_files/
+    /// is_dir answer through the path mapping, no namespace privileges
+    /// needed.
+    fn unshare_test_context(chroot_root: &Path) -> Context {
+        let base = Context::new(crate::context::ContextConfig::Local).unwrap();
+        Context::with_parent(
+            crate::context::ContextConfig::Unshare {
+                path: chroot_root.to_string_lossy().to_string(),
+                parent: None,
+            },
+            Arc::new(base),
+        )
+    }
+
+    /// The staging-area listing must classify entries through the context:
+    /// an unshare context returns build-root-relative paths that a host-side
+    /// stat never sees (they live under the chroot root on the host), which
+    /// used to silently empty the 'Found directories' list of the search
+    /// failure message — and with it every hint about the actual layout.
+    #[test]
+    fn find_package_directory_lists_staged_directories_through_the_context() {
+        let chroot = tempfile::tempdir().unwrap();
+        // Staged parent holding a single tree whose name matches none of
+        // the search patterns (the embedded-caller layout: <job>/tree)
+        let staged_parent = chroot.path().join("tmp/pkh-build-1/j-42");
+        std::fs::create_dir_all(staged_parent.join("tree/debian")).unwrap();
+
+        let ctx = unshare_test_context(chroot.path());
+        let err = find_package_directory(
+            Path::new("/tmp/pkh-build-1/j-42"),
+            "bc",
+            "1.07.1-1ubuntu1",
+            "questing",
+            &ctx,
+        )
+        .expect_err("no candidate matches a tree named 'tree'");
+
+        let message = err.to_string();
+        assert!(
+            message.contains("Found directories: tree"),
+            "error should list the staged directories through the context: {message}"
+        );
+    }
+
     async fn test_build_end_to_end(
         package: &str,
         series: &str,