Bug 23091: Make relations more explicit

Bug 23091: Make relations more explicit

This patch simply makes the queries built for the ->debits and ->credits
relations in Koha::Account::Line more explicit.

Test plan
1/ Ensure tests continue to pass
2/ Run tests with DBIC_TRACE=1 DBIC_TRACE_PROFILE=console and note that
the related queries not include 'credit.' and 'debit.' as appropriate.

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>