In the modular solver, it's now possible to mark packages
as disabled in the index conversion phase. This can be used
to mark broken packages (with missing dependencies) in such
a way that they can still be seen by the solver and reported
in error messages.

Furthermore, the feature could be used to implement
shadowing restrictions. Currently, there are some things
that can go wrong if multiple installed instances of
the same version and package exist on the system. However,
I've not enabled the use of shadowing for now, as it's
hard to match the restrictions exactly, and we can end up
excluding perfectly valid cases. Enabling shadowing
selectively by a solver flag would be an option.