Refactor the "_ROOT_DIR" logic into FindPackageHelper for CMake.

The unbundled libevent build has a TODO that recommends moving the logic for determining the root-dir of a package into our FindPackageHelper.
See https://github.com/apache/mesos/blob/ba0f78112b766e2a1d6e43c40d7fb2f7569c2580/3rdparty/cmake/FindLIBEVENT.cmake#L20-L40

That logic is pretty neat for e.g. homebrew users as it makes use of the installation prefix information gathered from the brew tool.

Instead of copy&pasting the same thing over and over into respective FindLIBxxx.cmake packages, we should move it up into FindPackageHelper.cmake.