Cleanup: localize creator.c definitions

As creator.c is used for the 'main' function,
avoid obscure declarations being placed so prominently.

- Move Blender as a Python module declarations into it's own section.
- Move USD declaration to an 'extern' just before it's called.

source/blender/python/intern/bpy_interface.c

@@ -921,8 +921,11 @@ int BPY_context_member_get(bContext *C, const char *member, bContextDataResult *
 /* TODO, reloading the module isn't functional at the moment. */
 
 static void bpy_module_free(void *mod);
+
+/* Defined in 'creator.c' when building as a Python module. */
 extern int main_python_enter(int argc, const char **argv);
 extern void main_python_exit(void);
+
 static struct PyModuleDef bpy_proxy_def = {
     PyModuleDef_HEAD_INIT,
     "bpy",           /* m_name */