This method is used to import ZODB Components when fallback on filesystem is needed
and used when loading Document and Extension Components.

1,000,000 executions:
  * Before:
    timeit("erp5.component.extension.find_load_module(module_name)",
           setup="import erp5.component.extension; module_name='Hoge'")
    59.073
    59.324
  * b45914fa ExternalMethod optimization:
    timeit("__import__('erp5.component.extension.' + module_name, fromlist='*', level=0)",
           setup="module_name='Hoge'")
    1.754
    1.845
  * This commit:
    timeit("import_module('erp5.component.extension.' + module_name)",
           setup="from importlib import import_module; module_name='Hoge'")
    1.346
    1.311
    => Faster because import_module() calls __import__() without fromlist (not
       needed as it returns module from sys.modules after this) and level
       parameters.
    => Introduced in python3 and backported to python2.7. Recommended over
       __import__ in Python documentation for non-internal usage and when
       getting the module itself and not its top-level one.