find_package: Add variable to make package REQUIRED

Add a `CMAKE_REQUIRE_FIND_PACKAGE_<PackageName>` variable is complement
to `CMAKE_DISABLE_FIND_PACKAGE_<PackageName>` with just the opposite
behaviour: it turns non-required find_package call into the required one.

While optional package dependencies usually result in simple and clean
build logic, sometimes people want to be sure those optional
dependencies will be found and used. Examples are reproducible builds
and build instructions for 3rd parties. People choose to make
find_package calls REQUIRED and put them behind an option(). Such
workarounds blend build logic with build environment management and do
not look elegant.