[MPMD-283] Create a real aggregate goal

[adangel]

This PR adds a couple of new goals for creating aggregated reports:

• aggregate-pmd
• aggregate-pmd-no-fork
• aggregate-pmd-check

This fixes MPMD-283

It uses the same logic as the javadoc plugin, e.g. the aggregate* goals create aggregated reports on every level and not just on the top level.

The old mechanism with the property aggregate=true is still preserved, but deprecated.

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