[MDEPLOY-173] support flat pom functionality in maven-deploy-plugin

Enhance the maven deploy plugin to deploy a "flattened" pom that contains only GAV and <dependencies> section.
The <dependencies> section contains, properly resolved direct dependencies of the artifact, which are the same of "pom hierarchy" direct dependencies.
"flattened" pom means it cannot have any parent or any other sections that are not necessary while deploying the artifact.