modules: get rid of WITH_FIND_MODULE, avoid modifying CMAKE_MODULE_PATH.

CMake modules are now *always* installed into
<prefix>/share/cmake/Corrade and FindCorrade.cmake is also always
installed, but it doesn't require root privileges anymore.

To keep things working, you need to update FindCorrade.cmake modules in
your projects and also replace include(CorradeLibSuffix) with
include(${CORRADE_LIB_SUFFIX_MODULE}), if used.
modules: simplified flag parsing in FindCorrade.cmake.