Uninstall packages in a chroot

[zlin]

cave resolve \!foo::chroot and cave uninstall foo::chroot just says "(nothing to do)".

[ciaranm]

The reason for this is:

const DestinationTypes
Decider::_get_destination_types_for_blocker(const BlockDepSpec &) const
{
    return DestinationTypes() + dt_install_to_slash;
}

We need to decide what that should really do. We could make it apply to both / and the chroot, if a chroot is present, but that's wrong for explicit ::repo blockers. Or we could make it follow the same logic that normal dependencies do, but that would result in most dep blockers being applied just to /. Or we could make it apply to the make destination for targets that aren't ::repo or ::/path qualified, and / plus the chroot for dependencies. Or something else...

[ciaranm]

Uninstalls now work using -mc blah, but not blah::inst.