Enhance modelling of Liberty modules to include multi-module Liberty projects.

[mrglavas]

This representation should include:

• the aggregator (parent) and submodule (child) relationships.
• the root modules, top-level aggregators and independent single module projects
• the leaf modules, e.g. the projects containing Liberty server configuration

This should be kept generic enough so that this model will continue to work with Gradle as-is and with future support for multi-module Gradle projects. See current LibertyModule and related classes for reference.