[SPARK-13296][SQL] Move UserDefinedFunction into sql.expressions.

This pull request has the following changes:

1. Moved UserDefinedFunction into expressions package. This is more consistent with how we structure the packages for window functions and UDAFs.

2. Moved UserDefinedPythonFunction into execution.python package, so we don't have a random private class in the top level sql package.

3. Move everything in execution/python.scala into the newly created execution.python package.

Most of the diffs are just straight copy-paste.

Author: Reynold Xin <rxin@databricks.com>

Closes #11181 from rxin/SPARK-13296.