Refactor facet information for a single field to a separate routine

Problem/Motivation

This issue is a precursor to a couple of other issues to be filed on this module and another solr-related module. In the course of implementing the other changes, it became evident that a refactor of apachesolr_entity_field_facets() to extract the inner loop code into a separate routine would allow that code to be reused by other modules.

This is a benign change (assuming no bug in what is in the patch). In other words, it does not change the operation of the existing code. It still loops on the search fields and sets the facet information array. It simply exposes the latter code for reuse.

Proposed resolution

Extract the inner loop code of apachesolr_entity_field_facets() into a separate routine.

Remaining tasks

Test to confirm claim of benign change.

User interface changes

None.

API changes

None.

Data model changes

None.