SPARK-3996: Shade Jetty in Spark deliverables.

This patch relocates Jetty classes inside of the assembly jar in Spark.
It does not modify Spark's disclosed dependency on Jetty, since we
already ask users to mark Spark and its dependencies as "provided"
anyways. This is a small fix that hopefully can be used as a template
for other locations in which we shade libraries.

This is a simpler version of #2984. If merged, this closes #2984.