[SPARK-16159][SQL] Move RDD creation logic from FileSourceStrategy.apply

## What changes were proposed in this pull request?
We embed partitioning logic in FileSourceStrategy.apply, making the function very long. This is a small refactoring to move it into its own functions. Eventually we would be able to move the partitioning functions into a physical operator, rather than doing it in physical planning.

## How was this patch tested?
This is a simple code move.

Author: Reynold Xin <rxin@databricks.com>

Closes #13862 from rxin/SPARK-16159.