gh-154675: Use lazy imports in the top-level modules of importlib

[brettcannon]

• Issue: Improve import time of various stdlib modules #154675

[hugovk]

No objection to tidying the imports here, but I don't see much if any time saved here: _imp is a builtin and already loaded, warnings was already deferred.

We could get some good savings from importlib.resources (15ms -> 8ms) and importlib.metadata (10ms -> 7ms), but should also consider the backports.