find_package: Use <PACKAGENAME>_ROOT variables as search prefixes

Extend commit eb35d8884b (find_package: Use PackageName_ROOT variables
as search prefixes, 2018-03-15, v3.12.0-rc1~349^2) to also check
upper-case `<PACKAGENAME>_ROOT` variables.  Add policy `CMP0144` to
enable the behavior in a compatible way.

Fixes: #24403