[MPMD-304] - maven-pmd-plugin should be toolchains-aware

[adangel]

This PR adds toolchain support for PMD and CPD (see https://issues.apache.org/jira/browse/MPMD-304).

In order to do this, the following changes are made:

• Execution is now separated from PmdReport/CpdReport and moved to PmdExecutor/CpdExecutor
• The properties/parameter are handed over as PmdRequest/CpdRequest
• If a toolchain is in use, then PmdExecutor/CpdExecutor is called in a forked JVM. PmdRequest/CpdRequest is serialized/deserialized.
• The result is provided for reporting via PmdResult/CpdResult. These classes are just wrappers around the always created target/pmd.xml and target/cpd.xml files. So basically, PMD serializeds the result into these XML files, and m-pmd-p picks it up from there.

There are two possible incompatibilities:

• The public method PmdReport.getPMDConfiguration() is gone - the PMDConfiguration instance is created now by PmdExecutor and is not exposed anymore.
• PmdReportGenerator and CpdReportGenerator now work with the classes generated by m-pmd-p's generated modello classes instead of the PMD classes.

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