[SPARK-17673][SQL] Incorrect exchange reuse with RowDataSourceScan

## What changes were proposed in this pull request?

It seems the equality check for reuse of `RowDataSourceScanExec` nodes doesn't respect the output schema. This can cause self-joins or unions over the same underlying data source to return incorrect results if they select different fields.

## How was this patch tested?

New unit test passes after the fix.

Author: Eric Liang <ekl@databricks.com>

Closes #15273 from ericl/spark-17673.