Fix pkg_tar directory entries

Directories appear to work for zip files but not tar files.

This should ensure we don't try to process directories as normal
files.

Fixes issues like:
rules_pkg/pkg/private/tar/tar_writer.py", line 242, in add_file
    with open(file_content, 'rb') as f:
IsADirectoryError: [Errno 21] Is a directory: 'bazel-out/darwin-fastbuild/bin/py/selenium/webdriver/common/devtools/v106'

pkg/private/tar/build_tar.py

@@ -309,7 +309,10 @@ def add_manifest_entry(self, entry_list, file_attributes):
     elif entry.entry_type == manifest.ENTRY_IS_EMPTY_FILE:
       self.add_empty_file(entry.dest, **attrs)
     else:
-      self.add_file(entry.src, entry.dest, **attrs)
+      if os.path.isdir(entry.src):
+        self.add_tree(entry.src, entry.dest, **attrs)
+      else:
+        self.add_file(entry.src, entry.dest, **attrs)
 
 
 def main():