Fix install path of module macros.

Markus Blattrequested to merge
feature/fix-module-install-in-am2cmake.py into master

According to @bernd.flemisch in /staging/dune-uggrid!21 (merged): The install directory has to be ${DUNE_INSTALL_MODULEDIR}. Otherwise, the file will not be found by dune_process_dependency_macros from DuneMacros.cmake.

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