If an optional package require further packages, those packages should
not be marked as REQUIRED because CMake will then terminate the
configuration if it is not found (although it is transitively optional).
Conversely, we should be able to specify REQUIRED prerequisites for
REQUIRED packages and have a failure cascade up to termination.
This changeset allows us to specify REQUIRED or not in the list of
prerequisites, and have OpmPackage sort of whether it should be honored
or not.
