[PDI-10846]: Refactored Kettle build/assemble to use Pentaho conventions

Tasks:

1) Standardized build targets (ant clean-all resolve dist) for every module, plugin, and the root
2) Publish core plugins as ZIPs, and resolve them like other CE plugins (big-data-plugin, e.g.)
3) Remove non-standard Subfloor targets, only override when necessary
4) Fully separate root-level build targets from assembly-level build targets
5) Re-enable source code package generation
6) Re-enable Cobertura code coverage using Kettle module unit tests
7) Verify integration test targets still run successfully