[SPARK-19092][SQL] Save() API of DataFrameWriter should not scan all the saved files

### What changes were proposed in this pull request?
`DataFrameWriter`'s [save() API](https://github.com/gatorsmile/spark/blob/5d38f09f47a767a342a0a8219c63efa2943b5d1f/sql/core/src/main/scala/org/apache/spark/sql/DataFrameWriter.scala#L207) is performing a unnecessary full filesystem scan for the saved files. The save() API is the most basic/core API in `DataFrameWriter`. We should avoid it.

The related PR: https://github.com/apache/spark/pull/16090

### How was this patch tested?
Updated the existing test cases.

Author: gatorsmile <gatorsmile@gmail.com>

Closes #16481 from gatorsmile/saveFileScan.