Rework FindPackageHelper to allow for locating xxx_ROOT_DIR.

With the introduction of unbundled libevent, libarchive and leveldb CMake builds, code duplication within the respective FindXXX.cmake modules became apparent and a refactoring seems adequate.

See this TODO: https://github.com/apache/mesos/blob/e6b507b6469553725c98b953f9578ab3f1a40b93/3rdparty/cmake/FindLIBARCHIVE.cmake#L19