repoman: Add check for missing slot values/slot operators

The idea if that a particular dependency atom matches more than one slot
of a package, you are supposed to either use := or :* operator
(or a specific :slot dependency), whichever is appropriate.

This will help catching mistakes (when packages become slotted) and make
cross-slot behavior clear (it is undefined with no slot operator). I
will estimate the amount of new warnings later.