gh-154675: reorganize some imports in 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.

[brettcannon]

_imp is a builtin and already loaded

Good catch! I've reverted it.

warnings was already deferred.

True, but it bugged me enough that I've been wanting to make this change to the import since lazy imports were announced, so I consider it clean-up along with making the re-exporting more obvious.

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

I view that as a call for @jaraco to make as that's mostly their code.