[SPARK-19960][CORE] Move `SparkHadoopWriter` to `internal/io/`

## What changes were proposed in this pull request?

This PR introduces the following changes:
1. Move `SparkHadoopWriter` to `core/internal/io/`, so that it's in the same directory with `SparkHadoopMapReduceWriter`;
2. Move `SparkHadoopWriterUtils` to a separated file.

After this PR is merged, we may consolidate `SparkHadoopWriter` and `SparkHadoopMapReduceWriter`, and make the new commit protocol support the old `mapred` package's committer;

## How was this patch tested?

Tested by existing test cases.

Author: jiangxingbo <jiangxb1987@gmail.com>

Closes #17304 from jiangxb1987/writer.