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
Getting rid of plexus.

After merge (this was in two modules), the module
became "halfling", as plugin was sisu but shared
code was still plexus. Plexus removed,
UTs update too
NEXUS-5784: Added UTs

Two UTs testing the changes:
* NPE does not occur in "inconsistent state"
* event inspector properly installs and uninstalls the catalog item
Removed redundant super() and NPEcheck
Removed redundant test.
Merge pull request #59 from sonatype/fix-archetype-npe

Fix NPE in archetype plugin.