Fix package conflicts formatting for CUDF resolver

For now, the formatting of the conflicts in the CUDF file (describing the universe of the resolution) is exactly the same as for regular dependencies.
This means that when a conflicting package version range is not present, the conflict is extended to all versions of this package instead of being removed.
Examples :

• For a dependency A:1.0.0:2.0.0, if neither 1.0.0 nor 2.0.0 are available, we end up with depends: A, which makes sense because any version of A is better than nothing (still a warning to the user is needed)
• For a conflict A:1.0.0:2.0.0, if neither 1.0.0 nor 2.0.0 are available, we end up with conflicts: A, which is wrong because we would like to be able to install any other version of A

Solution:
When the conflicting version range is not available in the CUDF universe, then set the conflict to a non-existing version instead of all versions (as we do for dependencies).
-> For the previous example, we should end up with conflicts: A = -2 which is basically removing the conflict