Update how importlib is imported (#825)

importlib's util module should be imported via import importlib.util not import importlib. The invalid import style can cause unexpected errors.

edk2toolext/environment/plugin_manager.py

@@ -7,7 +7,7 @@
 ##
 """This module contains code that supports Build Plugins."""
 
-import importlib
+import importlib.util
 import logging
 import os
 import sys