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.

Objects/exceptions.c

@@ -709,6 +709,13 @@ ComplexExtendsException(PyExc_Exception, ImportError,
                         "Import can't find module, or can't find name in "
                         "module.");
 
+/*
+ *    ModuleNotFoundError extends ImportError
+ */
+
+MiddlingExtendsException(PyExc_ImportError, ModuleNotFoundError, ImportError,
+                         "Module not found.");
+
 /*
  *    OSError extends Exception
  */
@@ -2395,6 +2402,7 @@ _PyExc_Init(PyObject *bltinmod)
     PRE_INIT(SystemExit)
     PRE_INIT(KeyboardInterrupt)
     PRE_INIT(ImportError)
+    PRE_INIT(ModuleNotFoundError)
     PRE_INIT(OSError)
     PRE_INIT(EOFError)
     PRE_INIT(RuntimeError)
@@ -2465,6 +2473,7 @@ _PyExc_Init(PyObject *bltinmod)
     POST_INIT(SystemExit)
     POST_INIT(KeyboardInterrupt)
     POST_INIT(ImportError)
+    POST_INIT(ModuleNotFoundError)
     POST_INIT(OSError)
     INIT_ALIAS(EnvironmentError, OSError)
     INIT_ALIAS(IOError, OSError)