Rename metrics-web packages

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

• Rename root package org.torproject.metrics to org.torproject.metrics.stats to make it part of the Tor Metrics name space.
• ernie.cron becomes org.torproject.metrics.stats.servers to get rid of the legacy name and to reflect that it's all about servers and bandwidth (provided by servers).
• org.torproject.metrics.web and subpackages remain unchanged.
• org.torproject.metrics.web.research, containing just a single class, is merged into org.torproject.metrics.web.