Bug 12896: Move the bookseller-related code into Koha::Acquisition::Bookseller

Bug 12896: Move the bookseller-related code into Koha::Acquisition::Bookseller

The C4::Acquisition module should be exploded in order to add
readability and maintainability to this part of the code.

This patch is a POC, it introduces a new Koha::Acquisition::Bookseller module and put in
it the code from GetBookSeller and GetBookSellerFromId.

Test plan:
1/ Create a bookseller, modify it.
2/ Add contacts for this bookseller
3/ Create an order, receive it, transfer it
4/ Launch the prove command on all unit tests modified by this patch and
verify that all pass.

Signed-off-by: Paola Rossi <paola.rossi@cineca.it>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>