archive: fix squashfs listings failing with invalid argument after update

The go-diskfs library now requires io/fs.ValidPath style paths (no
leading slash, "." for the root) so convert paths at the library
boundary. This fixes listing failures and the resulting test hangs
after the update to go-diskfs v1.9.3.

backend/archive/squashfs/squashfs.go

@@ -156,6 +156,17 @@ func (f *Fs) toNative(remote string) (string, error) {
 	return native, nil
 }
 
+// This turns a native path with a leading / into a path suitable for
+// the squashfs library which requires io/fs.ValidPath paths with no
+// leading / and "." for the root directory.
+func toIOFS(native string) string {
+	native = strings.Trim(native, "/")
+	if native == "" {
+		return "."
+	}
+	return native
+}
+
 // Turn a (nativeDir, leaf) into a remote
 func (f *Fs) fromNative(nativeDir string, leaf string) string {
 	// fs.Debugf(nil, "nativeDir = %q, leaf = %q, root=%q", nativeDir, leaf, f.root)
@@ -196,7 +207,7 @@ func (f *Fs) List(ctx context.Context, dir string) (entries fs.DirEntries, err e
 		return nil, err
 	}
 
-	items, err := f.sqfs.ReadDir(nativeDir)
+	items, err := f.sqfs.ReadDir(toIOFS(nativeDir))
 	if err != nil {
 		return nil, fmt.Errorf("read squashfs: couldn't read directory: %w", err)
 	}
@@ -238,7 +249,7 @@ func (f *Fs) newObjectNative(nativePath string) (o fs.Object, err error) {
 	leaf = strings.Trim(leaf, "/")
 
 	// FIXME need to detect directory not found
-	fis, err := f.sqfs.ReadDir(dir)
+	fis, err := f.sqfs.ReadDir(toIOFS(dir))
 	if err != nil {
 
 		return nil, fs.ErrorObjectNotFound