find_*: Disable the PACKAGE_ROOT search path group for CMake 3.9
Disable the feature added by !796 (merged). Unfortunately the name <pkg>_ROOT
may
already be set by projects for their own incompatible purposes. Disable
the behavior change for now to fix the regression for CMake 3.9. We can
restore it later with a policy.
In order to keep the implementation and tests working, add an undocumented variable we can use in the tests to enable the behavior before the policy is introduced.
Fixes: #17144 (closed)