Issue #15767: Introduce ModuleNotFoundError, a subclass of

ImportError.

The exception is raised by import when a module could not be found.
Technically this is defined as no viable loader could be found for the
specified module. This includes ``from ... import`` statements so that
the module usage is consistent for all situations where import
couldn't find what was requested.

This should allow for the common idiom of::

  try:
    import something
  except ImportError:
    pass

to be updated to using ModuleNotFoundError and not accidentally mask
ImportError messages that should propagate (e.g. issues with a
loader).

This work was driven by the fact that the ``from ... import``
statement needed to be able to tell the difference between an
ImportError that simply couldn't find a module (and thus silence the
exception so that ceval can raise it) and an ImportError that
represented an actual problem.

Lib/test/test_site.py

@@ -131,7 +131,7 @@ class HelperFunctionsTests(unittest.TestCase):
             re.escape(os.path.join(pth_dir, pth_fn)))
         # XXX: ditto previous XXX comment.
         self.assertRegex(err_out.getvalue(), 'Traceback')
-        self.assertRegex(err_out.getvalue(), 'ImportError')
+        self.assertRegex(err_out.getvalue(), 'ModuleNotFoundError')
 
     @unittest.skipIf(sys.platform == "win32", "Windows does not raise an "
                       "error for file paths containing null characters")