Add a validation that all packages in a module are exported

[zml2008]

For modules where all or almost all of the packages should be expocted, it would be helpful to have a task to automatically validate that every package that should be exported, is exported.

This task should be able to take a list of exceptions -- as a list of prefixes, or perhaps a regex (though probably the prefix list is more appropriate), so we can allow for a list of internal packages.