Find library debugging

This builds on !2313 (closed) (I hope you do not mind Chris Wilson).

.. though probably not in the best way. I am seeking advice on how to improve this.

It is such a useful feature for me. Recently I've been investigting why CMake is slow in some usecases in our ecosystem and this immediately shows me that find_library is looking in loads of directories that we do not want it to look into. I'm not sure pruning those will lead to a major speed ups but at least it will help greatly with isolation and reproducibility.