REVIEW [NX-485] remove last few embedded plugin dependencies

This patch leaves only two plugins with a couple of embedded dependencies, both of which are best kept:

 * nexus-extdirect-plugin - embeds directjngine which patches gson (easier to handle this with embedding)
 * nexus-p2-bridge-plugin - embeds Eclipse framework bootstrapping code (avoids conflict with Karaf/Felix)

The main issue in this PR was untangling the indexer and restlet1x plugins, because of the overlap between their respective REST models. We should investigate whether we can remove this overlap at the expense of potentially affecting compatibility with legacy REST clients, as it would make it easier to evolve the REST API.

https://issues.sonatype.org/browse/NX-485