Prepare for core allowing multiple taxonomy vocabularies on exposed filter

Problem/Motivation

There's a core issue related to allowing an exposed taxonomy filter to use terms from multiple vocabularies ( https://www.drupal.org/project/drupal/issues/2784233 ✨ Allow multiple vocabularies in the taxonomy filter Needs work ). This issue is currently set at RTBC.

When the RTBC patch is applied (#193), the core default exposed widget (select list) works with multiple vocabularies. However, the better_exposed_filter widgets do not work when a taxonomy filter is using multiple vocabularies.

Steps to reproduce

1. Apply the patch from the referenced issue.
2. Configure an exposed taxonomy term filter to show terms from multiple vocabularies.
3. Configure the exposed filter to use the Radio/Checkboxes plugin.
4. Note that when rendered, the exposed filter uses the index of the radio element as the value instead of using the term's target id as the value.

Proposed resolution

I think that the bef community needs to keep an eye on the referenced issue, maybe with the goal of quickly adding support once that issue is marked fixed.

Remaining tasks

Come up with a plan.

User interface changes

TBD

API changes

TBD

Data model changes

TBD