[SPARK-7899] [PYSPARK] Fix Python 3 pyspark/sql/types module conflict

This PR makes the types module in `pyspark/sql/types` work with pylint static analysis by removing the dynamic naming of the `pyspark/sql/_types` module to `pyspark/sql/types`.

Tests are now loaded using `$PYSPARK_DRIVER_PYTHON -m module` rather than `$PYSPARK_DRIVER_PYTHON module.py`. The old method adds the location of `module.py` to `sys.path`, so this change prevents accidental use of relative paths in Python.

Author: Michael Nazario <mnazario@palantir.com>

Closes #6439 from mnazario/feature/SPARK-7899 and squashes the following commits:

366ef30 [Michael Nazario] Remove hack on random.py
bb8b04d [Michael Nazario] Make doctests consistent with other tests
6ee4f75 [Michael Nazario] Change test scripts to use "-m"
673528f [Michael Nazario] Move _types back to types