Representation of possible resolution results of a package.
This holds three attributes:
* `information` is a collection of `RequirementInformation` pairs.
Each pair is a requirement contributing to this criterion, and the
candidate that provides the requirement.
* `incompatibilities` is a collection of all known not-to-work candidates
to exclude from consideration.
* `candidates` is a collection containing all possible candidates deducted
from the union of contributing requirements and known incompatibilities.
It should never be empty, except when the criterion is an attribute of a
raised `RequirementsConflicted` (in which case it is always empty).
.. note::
This class is intended to be externally immutable. **Do not** mutate
any of its attribute containers.