Merge duplicated readdir functionality

internal/archiver.readdir and internal/fs.ReadDir were unused.

internal/fs.ReadDirNames and internal/archiver.readdirnames were doing
nearly the same thing, except one sorted its output and opened with
fs.O_NOFOLLOW. Both were only used in internal/archiver.

internal/archiver/tree.go

@@ -214,7 +214,7 @@ func unrollTree(f fs.FS, t *Tree) error {
 	// nodes, add the contents of Path to the nodes.
 	if t.Path != "" && len(t.Nodes) > 0 {
 		debug.Log("resolve path %v", t.Path)
-		entries, err := fs.ReadDirNames(f, t.Path)
+		entries, err := readdirnames(f, t.Path, 0)
 		if err != nil {
 			return err
 		}