Rewrite FindYamlCpp.cmake.

It now follows CMake conventions better, respects REQUIRED and SILENT
(by using FPHSA), and is much simpler.

Windows complexity to come later :D
Use the correct CMake find-module variables.

XXX_INCLUDE_DIR -> XXX_INCLUDE_DIRS
XXX_LIBRARY -> XXX_LIBRARIES

For example, on Windows, protobuf sets PROTOBUF_LIBRARY to the release
library, and sets PROTOBUF_LIBRARIES to a more complex expression that
uses either the debug or release library as appropriate, so we should use
the latter.