[SPARK-20594][SQL] The staging directory should be a child directory starts...

[SPARK-20594][SQL] The staging directory should be a child directory starts with "." to avoid being deleted if we set hive.exec.stagingdir under the table directory.

JIRA Issue: https://issues.apache.org/jira/browse/SPARK-20594

## What changes were proposed in this pull request?

The staging directory should be a child directory starts with "." to avoid being deleted before moving staging directory to table directory if we set hive.exec.stagingdir under the table directory.

## How was this patch tested?

Added unit tests

Author: zuotingbing <zuo.tingbing9@zte.com.cn>

Closes #17858 from zuotingbing/spark-stagingdir.