CSV Import: allow using ignored fields as slug

[madebyisaacr]

Description

This pull request updates the CSV import plugin to allow using fields that are ignored/disabled as the slug field. This fixes an issue where if your CSV has a slug column, it would create duplicate fields because it would import as both the slug value and as a separate field.

I also made it check if there's a column that matches the slug field name, and if so it will be selected by default and the row will be ignored by default to avoid creating a duplicate slug field.

Testing

Google Sheet for testing: https://docs.google.com/spreadsheets/d/1JuOKh_Wb0uIytKe4eE7uI6Js826i8V_uf2oFtyolDtA/edit?gid=1174080602#gid=1174080602

• Check if the "Slug" column is the default selected slug field even though it is not the first column.
• Disable the "Slug" column and check if it still allows you to use it as the slug field.