feat: disable candidates

[baszalmstra]

Adds the ability to disable specific candidates within the solver. This can be useful for error reporting.

Previously one would filter out incompatible candidates before passing them to the solver. This has the downside that if the problem is unsolvable the filtering is not transparent to the user. With this change the solver receives the candidates and immediately deselects them. However, they are still present in the error output with a custom message:

The following packages are incompatible
|-- a * cannot be installed because there are no viable options:
    |-- a 1 is disabled because it is externally disabled

The it is externally disabled in this example was injected into the solver.

I think in a next iteration it would be nice to merge some of these problems together to get a more readable output. This is currently a bit verbose if there are many disabled versions.

[wolfv]

Just a little typo. Looks great to me, excited for this!