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.