[SPARK-9791] [PACKAGE] Change private class to private class to prevent...

[SPARK-9791] [PACKAGE] Change private class to private class to prevent unnecessary classes from showing up in the docs

In addition, some random cleanup of import ordering

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #8387 from tdas/SPARK-9791 and squashes the following commits:

67f3ee9 [Tathagata Das] Change private class to private[package] class to prevent them from showing up in the docs