Set standard variables for package after probing

Standard variables must be set in order for the package to be picked
up with the OpmFind/OpmPackage dance.
OPM-Material module must have DUNE Common and DUNE ISTL
Only search for a package again if not found

If it is already found, most packages sets a flag that makes it exit
immediately anyway. We usually cannot rerun find_package to get another
combinations of modules. Here we check the flag before we start looking
for the package to avoid the endless stream of messages.
Propagate REQUIRED and QUIET according to parent level

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.
Merge pull request #322 from rolk/322_req

Propagate "requiredness" to prerequisites depending on the package's level