Bug 22701: Make items prefetchable from Koha::Biblio

Bug 22701: Make items prefetchable from Koha::Biblio

This patch makes Koha::Biblio->items use _new_from_dbic instead of
calling Koha::Items->search. This allows calling it with prefetch
enabled for the items relationship. No noticeable behaviour change takes
place besides this.

To test:
- Run:
  $ kshell
 k$ prove t/db_dependent/Koha/Biblio.t
=> SUCCESS: Tests pass!
- Apply this patch
- Run:
 k$ prove t/db_dependent/Koha/Biblio.t
=> SUCCESS: Tests still pass!
- Sign off :-D

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>