gh-117953: Cleanups For fix_up_extension() in import.c (gh-118192)

gh-117953: Cleanups For fix_up_extension() in import.c (gh-118192)

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.