Since jsoncpp 0.7.0 (2014-11-20) the upstream may provide a CMake
package configuration file such that find_package(jsoncpp) will find a
jsoncppConfig.cmake file.  In order to avoid conflicting with this
(especially on case-insensitive filesystems), and since we always prefer
projects to provide package config files (that they maintain), it is
better to not provide FindJsonCpp publicly.

Move FindJsonCpp into a private source directory that is not installed
so that we can still use it for building CMake itself.

Reported-by: Ryan Pavlik <ryan.pavlik@gmail.com>