[BUGFIX] modified source loop

* in case of file_per_set and pk_autoincrement, we need to loop
  over classes in config->{sets}, because related elements are
  generated implicit and if we would loop over all classes it would
  generate error because we would try to create these elements
  explicitly.

 * If there is a unique key field, it bails out with an error
 * If no unique key, the element is generate silently twice, which
   is also wrong (- but more confusing)
Merge branch 'master' of github.com:M0ses/DBIx-Class-Fixtures