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.

Python/sysmodule.c

@@ -3764,7 +3764,7 @@ _PySys_Create(PyThreadState *tstate, PyObject **sysmod_p)
         return status;
     }
 
-    if (_PyImport_FixupBuiltin(sysmod, "sys", modules) < 0) {
+    if (_PyImport_FixupBuiltin(tstate, sysmod, "sys", modules) < 0) {
         goto error;
     }