Add composite search filter ls:library

[kwahlin]

Refactor how libraries are queried:

• Downgrade ls:itemHeldBy and ls:itemHeldByOrg to auxiliary sub-properties.
• Introduce ls:library as a composite super-property, now used as the primary filter for searching both Library and bibdb:Organization.
• Mark the sub-properties with :category :coercing to mirror the behavior of ls:workCategory and its sub-properties.
• Retain ls:itemHeldByOrg in facets where results should be limited to organizations.
• Adjustments in XL: Feature/library search filter coercion librisxl#1754
• Minor adjustment in API tests: Use library (new search filter) instead of itemHeldBy in test lxl_api_tests#47