NEXUS-5525: Custom packaging types do not show up in index after deployment

Tracking down the problem revealed a lack of changes when MI
indexer-artifact was moved out from core. Since 2.2 or 2.1,
when indexer-artifact part of MI was moved out from core (indexer-core
was moved even earlier), the package mapper had to be moved into Maven
Support of Nexus as other parts of Nexus did rely on it. But, MI code
still relied on the package mapper from MI.

But since the move, nothing was setting it up properly, hence
all the packaging mappings were left unused (not picked up)
in the process of indexing. Due to this, artifact locator
was unable to locate the artifact if POM file was given.

This change now introduces a "wrapper" plexus component that
redirects MI package mapper to Nexus package mapper (they
are basically same but in different packages).
NEXUS-5525: UT covering the change.
Merge pull request #777 from sonatype/nexus-5525

NEXUS-5525: Custom packaging types do not show up in index after deployment