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.

Author: ericsnowcurrently

Include/internal/pycore_import.h

@@ -24,10 +24,12 @@ extern int _PyImport_ReleaseLock(PyInterpreterState *interp);
 
 // This is used exclusively for the sys and builtins modules:
 extern int _PyImport_FixupBuiltin(
+    PyThreadState *tstate,
     PyObject *mod,
     const char *name,            /* UTF-8 encoded string */
     PyObject *modules
     );
+// We could probably drop this:
 extern int _PyImport_FixupExtensionObject(PyObject*, PyObject *,
                                           PyObject *, PyObject *);