Fix NPE in archetype plugin.

Related to
https://issues.sonatype.org/browse/PR-4758

Problem was multiple:
* no NPE protection, but model might be null after Indexer
  changes (see XXX Igor's comment).
* Archetype plugin never properly "uninstalled" the
  generated item from repo root, only installed

Also, as before collapse this plugin was actually two modules
(one outside nx and one in nx), one was SISUized
while another remained Plexus. This is cleaned up
as well along with UT hierarchy and use of Plx.

Issue
https://issues.sonatype.org/browse/NEXUS-5784

CI
https://bamboo.zion.sonatype.com/browse/NX-OSSF13