find_package: Option to use <PackageName>_ROOT exclusively
CMake 3.12 introduced CMP0074 to enable use of <PackageName>_ROOT
variables as search prefixes. They are given the highest precedence. However, searches still move on to locations outside the named root.
In some use cases one just wants everything to be found under a given root and artifacts outside that root to be ignored even if they might otherwise be found. An additional setting will need to be designed to enable this.