[cxxmodules] Respect LD_LIBRARY_PATH order and save memory

std::set was not good because it sorted libraries by alphabetical order
and didn't preserve LD_LIBRARY_PATH order information. We can use
std::vector instead. (Erasing rarely happens and doesn't cost so much)

Path prefix like "/home/yuka/modules/lib/" costs memory, I
think it makes sense to avoid path duplication in sLibraries and make sPaths
vector for this prefix.