Follow-up to #6987: Bunch of prominent contrib break with change to _field_filter_items()

[herbdool]

Description of the bug

Bunch of contrib modules are affected by #6987.

Yes, it's a private function, but a bunch of prominent modules are using it like a public one. https://github.com/search?q=org%3Abackdrop-contrib+_field_filter_items&type=code

We could make it a bit easier by making $instance optional in _field_filter_items(). I'm wondering if we also make a version of that function public so we know not to touch it without making sure it's backwards-compatible in the future.

[indigoxela]

I'm wondering if we also make a version of that function public so we know not to touch it without making sure it's backwards-compatible in the future.

Following contrib modules use (not so private) _field_filter_items():

• feeds
• multifield
• unique_field
• paragraphs
• conditional_fields
• field_visibility_condition
• entity_translation

IMO that indicates that there's actually a need for such a utility function - public. That might be a follow-up, though.

The question, whether to fix a "private" function in core to not break contrib (or custom) is a little hard to answer... Luckily I don't have to decide. 😉

IF it's a fatal ArgumentCountError, I'm leaning towards fixing it in core - with a comment that we put that straight in Backdrop 2.x.

Edit: does it actually break contrib projects, though? I tried to reproduce with an older version of paragraphs and also with conditional_fields, but wasn't able to provoke a fatal... (Note, with the other PR from #7057 applied.)