OpmFind.cmake: prevent multiple calls to the same module in the same run

this prevents to check for the same software package more than once in the same cmake run and should thus speed things up a bit and make the terminal output a bit cleaner. For this, I assumed that the ${name}_FOUND cmake variable does not get cached and that if a package is found or not found the first time, the module produces the same result for all consecutive calls. It looks like all modules in opm-core exhibit these properties...