Fix importlib import in shared plugins manager

[jedcunningham]

This was only being imported if load_examples was on.

Before:

Traceback (most recent call last):
  File "/Users/jedc/github/airflow/airflow-core/src/airflow/_shared/plugins_manager/plugins_manager.py", line 234, in _load_plugins_from_plugin_directory
    loader = importlib.machinery.SourceFileLoader(mod_name, file_path)
             ^^^^^^^^^
UnboundLocalError: cannot access local variable 'importlib' where it is not associated with a value

[jedcunningham]

Duplicate of #60373