Exclude mavis module in module_wants_mavis/1

The mavis module never wants mavis types.  Make this clear by moving the
module exclusion into the convenienc predicate module_wants_mavis/1
Convenient debugging message for modelines

When debugging modelines, this message proved helpful.  I don't want its
output clogging the display right now, but want to keep it around as a
reminder.
Eagerly load necessary modules

I thought I could lazily load the quickcheck and error modules inside
the couple predicates that needed them.  That causes trouble with
list_undefined/0.  The laziness was probably a premature optimization.

Now that quickcheck is a mandatory dependency, list it in pack.pl too.