Added CMAKE_PREFIX_PATH to the module file
    
    ... to unsure that CMake projects find the available MPI enabled
    packages, libraries, executables, etc.
    
    The motivation is that CMake projects will search first in the
    CMAKE_PREFIX_PATH, both for cmake specific files like <Package>Config.cmake as
    well as libraries and header files in general. Adding this variable is crucial
    to making sure that the MPI versions take precedence over the serial versions.