find_*: ensure consistent behavior for cache variables

Help/manual/cmake-policies.7.rst

Help/policy/CMP0125.rst

Help/release/dev/find_item-consistent-behavior.rst

Source/cmFindBase.cxx

Source/cmMakefile.cxx

Source/cmPolicies.h

Tests/RunCMake/CMP0125/CMP0125-find_file-Common.cmake

Tests/RunCMake/CMP0125/CMP0125-find_file-NEW-stderr.txt

Tests/RunCMake/CMP0125/CMP0125-find_file-NEW.cmake

Tests/RunCMake/CMP0125/CMP0125-find_file-OLD-stderr.txt

Tests/RunCMake/CMP0125/CMP0125-find_file-OLD.cmake

Tests/RunCMake/CMP0125/CMP0125-find_library-Common.cmake

Tests/RunCMake/CMP0125/CMP0125-find_library-NEW-stderr.txt

Tests/RunCMake/CMP0125/CMP0125-find_library-NEW.cmake

Tests/RunCMake/CMP0125/CMP0125-find_library-OLD-stderr.txt

Tests/RunCMake/CMP0125/CMP0125-find_library-OLD.cmake

Tests/RunCMake/CMP0125/CMP0125-find_path-Common.cmake

Tests/RunCMake/CMP0125/CMP0125-find_path-NEW-stderr.txt

Tests/RunCMake/CMP0125/CMP0125-find_path-NEW.cmake

Tests/RunCMake/CMP0125/CMP0125-find_path-OLD-stderr.txt