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

Include/internal/pycore_runtime_init.h

@@ -97,23 +97,10 @@ extern "C" {
         ._main_interpreter = _PyInterpreterState_INIT, \
     }
 
-#ifdef HAVE_DLOPEN
-#  include <dlfcn.h>
-#  if HAVE_DECL_RTLD_NOW
-#    define _Py_DLOPEN_FLAGS RTLD_NOW
-#  else
-#    define _Py_DLOPEN_FLAGS RTLD_LAZY
-#  endif
-#  define DLOPENFLAGS_INIT .dlopenflags = _Py_DLOPEN_FLAGS,
-#else
-#  define _Py_DLOPEN_FLAGS 0
-#  define DLOPENFLAGS_INIT
-#endif
-
 #define _PyInterpreterState_INIT \
     { \
         .id_refcount = -1, \
-        DLOPENFLAGS_INIT \
+        .imports = IMPORTS_INIT, \
         .ceval = { \
             .recursion_limit = Py_DEFAULT_RECURSION_LIMIT, \
         }, \