CMake find_package requires CONFIG mode

[mkruskal-google]

The FindProtobuf module has shipped with cmake since version 3.6, but it is no longer compatible with our cmake setup.

If you use module mode of find_package by typing something like:

find_package(Protobuf)

You will end up with a bad configuration and a broken build. To get this to work you must use config mode like:

find_package(Protobuf CONFIG)

which will use the cmake configs we ship with our package.