Deprecate file.field.inc and move functions to file.module

Problem/Motivation

file.module calls:

// Load all Field module hooks for File.
require_once __DIR__ . '/file.field.inc'; 

However, file.field.inc only contains two template preprocess functions (template_preprocess_file_widget_multiple() and template_preprocess_file_upload_help()) and an 'internal' function file_field_find_file_reference_column that is only used by file_get_file_references in file.module.

Steps to reproduce

Proposed resolution

Move functions from file.field.inc to file.module and deprecate according to https://www.drupal.org/about/core/policies/core-change-policies/drupal-d...

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet