Rework the dependencies definition to include a 'required' flags, which
is passed by the check_dependencies helper to the callers, so that they
can distinguish between required and optional dependencies.
This help in two ways:
- The unit test for the dependencies was now failing when an optional
one was missing, due to a previous rework.
- Mat2's --check-dependencies was referring to "required dependencies"
and was misleading for the user as some of them could be optional.
Signed-off-by: Antoine Tenart <antoine.tenart@ack.tf>
f19f6ed8b6