cmake: Simplify and improve FindGraphviz.cmake

This has been sitting in Phab for 23 months. It is an extension of what FreeBSD ships as packaging patches (because otherwise we don't find GraphViz & dot anyway), and Works For Me against current master. Note that there are other things to fix in FindGraphviz with CMake 3.17, because the module name doesn't match the variables it defines (namely, GraphViz with different capitalization).

Differential Revision: https://phabricator.kde.org/D14146