title: What is the difference between `sys.meta_path` and `sys.path_hooks` importer objects?
body: <p>Using <code>importlib</code>, what is the difference between "Meta Path Finder" (found by traversing over sys.meta_path) and Path Entry Finder" (found by traversing over sys.path_hooks)?</p>
  
  <p>The first type is called upon begin of an import, but when is the second type used? Do both return a spec object? </p>
  
  <p>I want to implement a customized import, where a module can be imported from sources other than *.py or *.pyc, e.g. from a stream. How can this be done?</p>