Integrate PHPMD

[TerryKern]

As this is now a self contained open-sourced repository we should integrate PHPMD.
PHPMD short for 'PHP mess detector' and is used to identify 'messy' or duplicated code.

More information:

• https://github.com/phpmd/phpmd

[Neluxx]

This dependency is not strictly necessary and will be replaced by phpstan #19.