Bug 29786: Select only specific items for item level holds

Bug 29786: Select only specific items for item level holds

This patch adjusts get_items_that_can_fill to make two requests:
first the list of items for item-level holds
second the list of biblionumbers for title-level holds

This stops the report from pulling more items for item-level hold

This patch also removes the aliases used in the code - while readability
is a bit harder, it allows for using 'me' in get_items_that_can_fill
Otherwise, this routine would need a parameter to know what we called the table.

To test:
1 - Find a record with many items available
2 - Place an item level hold for an item on the record, not the one with lowest itemnumber
3 - Run 'Hold to pull' report
4 - Note the barcode does not match
5 - Apply patch
6 - Reload report
7 - It matches!

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>