gh-101758: Clean Up Uses of Import State (gh-101919)

This change is almost entirely moving code around and hiding import state behind internal API.  We introduce no changes to behavior, nor to non-internal API.  (Since there was already going to be a lot of churn, I took this as an opportunity to re-organize import.c into topically-grouped sections of code.)  The motivation is to simplify a number of upcoming changes.

Specific changes:

* move existing import-related code to import.c, wherever possible
* add internal API for interacting with import state (both global and per-interpreter)
* use only API outside of import.c (to limit churn there when changing the location, etc.)
* consolidate the import-related state of PyInterpreterState into a single struct field (this changes layout slightly)
* add macros for import state in import.c (to simplify changing the location)
* group code in import.c into sections
*remove _PyState_AddModule()

https://github.com/python/cpython/issues/101758

Python/dynload_shlib.c

@@ -75,7 +75,7 @@ _PyImport_FindSharedFuncptr(const char *prefix,
             return NULL;
     }
 
-    dlopenflags = _PyInterpreterState_GET()->dlopenflags;
+    dlopenflags = _PyImport_GetDLOpenFlags(_PyInterpreterState_GET());
 
     handle = dlopen(pathname, dlopenflags);