bpo-32030: Move PYTHONPATH to _PyMainInterpreterConfig (#4511)

Move _PyCoreConfig.module_search_path_env to _PyMainInterpreterConfig
structure.
diff --git a/Include/pystate.h b/Include/pystate.h
index c5d3c33..b2739f1 100644
--- a/Include/pystate.h
+++ b/Include/pystate.h
@@ -30,7 +30,6 @@
     unsigned long hash_seed;
     int _disable_importlib; /* Needed by freeze_importlib */
     const char *allocator;  /* Memory allocator: _PyMem_SetupAllocators() */
-    wchar_t *module_search_path_env; /* PYTHONPATH environment variable */
     int dev_mode;           /* -X dev */
     int faulthandler;       /* -X faulthandler */
     int tracemalloc;        /* -X tracemalloc=N */
@@ -46,7 +45,6 @@
      .hash_seed = 0, \
      ._disable_importlib = 0, \
      .allocator = NULL, \
-     .module_search_path_env = NULL, \
      .dev_mode = 0, \
      .faulthandler = 0, \
      .tracemalloc = 0, \
@@ -62,11 +60,13 @@
  */
 typedef struct {
     int install_signal_handlers;
+    wchar_t *module_search_path_env; /* PYTHONPATH environment variable */
 } _PyMainInterpreterConfig;
 
 #define _PyMainInterpreterConfig_INIT \
     (_PyMainInterpreterConfig){\
-     .install_signal_handlers = -1}
+     .install_signal_handlers = -1, \
+     .module_search_path_env = NULL}
 
 typedef struct _is {