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_import.py

@@ -68,7 +68,15 @@ class ImportTests(unittest.TestCase):
     def tearDown(self):
         unload(TESTFN)
 
-    setUp = tearDown
+    def test_import_raises_ModuleNotFoundError(self):
+        with self.assertRaises(ModuleNotFoundError):
+            import something_that_should_not_exist_anywhere
+
+    def test_from_import_raises_ModuleNotFoundError(self):
+        with self.assertRaises(ModuleNotFoundError):
+            from something_that_should_not_exist_anywhere import blah
+        with self.assertRaises(ModuleNotFoundError):
+            from importlib import something_that_should_not_exist_anywhere
 
     def test_case_sensitivity(self):
         # Brief digression to test that import is case-sensitive:  if we got
@@ -487,7 +495,7 @@ func_filename = func.__code__.co_filename
             header = f.read(12)
             code = marshal.load(f)
         constants = list(code.co_consts)
-        foreign_code = test_main.__code__
+        foreign_code = importlib.import_module.__code__
         pos = constants.index(1)
         constants[pos] = foreign_code
         code = type(code)(code.co_argcount, code.co_kwonlyargcount,
@@ -1014,16 +1022,5 @@ class ImportTracebackTests(unittest.TestCase):
             importlib.SourceLoader.load_module = old_load_module
 
 
-def test_main(verbose=None):
-    run_unittest(ImportTests, PycacheTests, FilePermissionTests,
-                 PycRewritingTests, PathsTests, RelativeImportTests,
-                 OverridingImportBuiltinTests,
-                 ImportlibBootstrapTests,
-                 TestSymbolicallyLinkedPackage,
-                 ImportTracebackTests)
-
-
 if __name__ == '__main__':
-    # Test needs to be a package, so we can do relative imports.
-    from test.test_import import test_main
-    test_main()
+    unittest.main()