Bug 24440: (follow-up) Make related objects prefetchable

Bug 24440: (follow-up) Make related objects prefetchable

This patch creates aliases for the relations used to retrieve the linked
objects from other tables. It makes the accessor names match the used
relation name, for consistency.

This is important for code trying to be smart and guessing what needs to
be prefetched, like the API code.

To test:
1. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Acquisition/Order.t
=> SUCCESS: Tests pass
2. Apply this patch
3. Repeat (1)
=> SUCCESS: Tests pass! No behaviour change
4. Verify the POD changes make sense
5. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>