> One (poorly thought out) option is to add sys._stdlib_path with the subsection of sys.path that contains the stdlib, and a function in importlib that returns if a spec is for a stdlib module.

There is already sysconfig.get_paths()['stdlib']. Maybe we need to add a new key for extension modules.

I don't think that these two options are exclusive. For me, it can even be a similar but different use case.