[NEXUS-6027] remove IOUtils
[NEXUS-6027] remove NexusPlugin, RestResource
deprecate NexusPluginManager
deprecate all plugins.* stuff this is all going to change in the future
[NEXUS-6027] remove unused org.sonatype.nexus.plugins.** cruft
[NEXUS-6027] remove org/sonatype/nexus/index/events/ReindexRepositories*