Prefix backtracking message with "INFO: "

Author: pradyunsg

src/pip/_internal/resolution/resolvelib/reporter.py

@@ -47,4 +47,4 @@ def backtracking(self, candidate):
             return
 
         message = self._messages_at_backtrack[count]
-        logger.info(message)
+        logger.info("INFO: %s", message)