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

this prevents to check for the same module more than once in the same
cmake run and should thus speed things up a bit as well as make the
output a bit cleaner. For this I assumed that the ${name}_FOUND cmake
variable does not get cached, which it seems like it does not...
OpmFind.cmake: also check for CAPITALIZED found variables

thanks to @rolk for catching this
Merge pull request #448 from andlaus/prevent_multiple_package_detections

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