gh-117953: Cleanups For fix_up_extension() in import.c

[ericsnowcurrently]

These are cleanups I've pulled out of gh-118116. Mostly, this change moves code around to align with some future changes and to improve clarity a little. There is one very small change in behavior: we now add the module to the per-interpreter caches after updating the global state, rather than before.

• Issue: Single-Phase Init Extension Module Init Functions Still Run in Isolated Interpreters #117953