Inconsistent handling of POMs

As we learned from discussion happened in issue MNG-4368, Maven3 – depending on packaging ("pom" vs anything else) – handles the POM installation in different ways.

When packaging is "pom", the POM is handled as artifact, and MNG-4368 (and any other future "optimization") interferes with it's installation. When packaging is not "pom", the POM is _handled as repository metadata in Maven3 internals.

IMHO, the latter should be always the case, regardless of the packaging type.