Share code to show pre-remove/post-install msgs.

include/xbps_api_impl.h

@@ -141,6 +141,12 @@ char HIDDEN *xbps_repository_pkg_path(struct xbps_handle *, xbps_dictionary_t);
 void HIDDEN xbps_fetch_set_cache_connection(int, int);
 void HIDDEN xbps_fetch_unset_cache_connection(void);
 
+/**
+ * @private
+ * From lib/package_msg.c
+ */
+int HIDDEN xbps_cb_message(struct xbps_handle *, xbps_dictionary_t, const char *);
+
 /**
  * @private
  * From lib/package_config_files.c