Rename ExoneraTor packages

This ticket is based on a discussion on #24036 (moved):

• Rename root package org.torproject.exonerator to org.torproject.metrics.exonerator to make it part of the Tor Metrics name space.
• Reconsider moving some classes to metrics-lib for re-use from metrics-web, but not before the Jetty switch and possibly related changes are over.