This commit fixes a subtle bug that may occur when
a package is a "possible_provider" of a virtual but
no "provides_virtual" can be deduced. In that case
the cardinality constraint on "provides_virtual"
may arbitrarily assign a package the role of provider
even if the constraints for it to be one are not fulfilled.
The fix reworks the logic around three concepts:
- "possible_provider": a package may provide a virtual if some constraints are met
- "provides_virtual": a package meet the constraints to provide a virtual
- "provider": a package selected to provide a virtual
acc11f676d