Bug 22685: Use DBIx::Class relationships in Koha::Acq::Bookseller methods

Bug 22685: Use DBIx::Class relationships in Koha::Acq::Bookseller methods

To match PERL15 we change the behavior of Koha::Object methods to make
them use the DBIx::Class relationships.

Test plan:
- Create a vendor, basket and contact
- Edit the vendor => the contact must appears
- Go to the view of a vendor => you cannot delete the vendor, it has
a basket attached
- Create another vendor, without basket
- Go to the view of the vendor => You can delete it

Additionally:
 % prove t/db_dependent/Koha/Acquisition/Booksellers.t
must return green

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>