py: Move weak-link map to objmodule.c, and expose module maps as public.

py/objmodule.h

@@ -28,6 +28,9 @@
 
 #include "py/obj.h"
 
+extern const mp_map_t mp_builtin_module_map;
+extern const mp_map_t mp_builtin_module_weak_links_map;
+
 void mp_module_init(void);
 void mp_module_deinit(void);
 mp_obj_t mp_module_get(qstr module_name);