Ignore fields with __prefix__ in id

[leibowitz]

django-smart-selects is currently initialising fields that aren't displayed on screen (including __prefix__ in the id)
https://docs.djangoproject.com/en/3.2/topics/forms/formsets/#empty-form

These are usually created for one/many-to-many relationships, and are cloned when a user clicks on the "add another" within the admin. The __prefix__ is then replaced with an index number (starting at 0)

There is a lot of code spread around to skip fields that include __prefix__:
https://github.com/django/django/blob/ca9872905559026af82000e46cde6f7dedc897b6/django/contrib/admin/static/admin/js/SelectFilter2.js#L11-L14
https://github.com/django/django/blob/ca9872905559026af82000e46cde6f7dedc897b6/django/contrib/admin/static/admin/js/autocomplete.js#L33

[codecov]

Codecov Report

Merging #332 (ed45b07) into master (2cff847) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #332   +/-   ##
=======================================
  Coverage   80.42%   80.42%           
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  Files           8        8           
  Lines         470      470           
  Branches       68       68           
=======================================
  Hits          378      378           
  Misses         60       60           
  Partials       32       32           

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