[SPARK-13118][SQL] Expression encoding for optional synthetic classes

## What changes were proposed in this pull request?

Fix expression generation for optional types.
Standard Java reflection causes issues when dealing with synthetic Scala objects (things that do not map to Java and thus contain a dollar sign in their name). This patch introduces Scala reflection in such cases.

This patch also adds a regression test for Dataset's handling of classes defined in package objects (which was the initial purpose of this PR).

## How was this patch tested?
A new test in ExpressionEncoderSuite that tests optional inner classes and a regression test for Dataset's handling of package objects.

Author: Jakob Odersky <jakob@odersky.com>

Closes #11708 from jodersky/SPARK-13118-package-objects.