duplicates: Overhaul commandline options handling

We noticed possible bugs caused by incomplete command line argument handling. A refactoring and better testing of the duplicates plugin would be in order.

Original post:
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It looks a bit like the plugin still isn't properly handling conflicting action options (what happens if `--delete` and `--tag` are given? Calling `item.store()` after `item.remove()` looks a little concerning). Probably, (some) actions should be made exclusive (at least upon executing them, maybe also warning the user about it).

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_Originally posted by @wisp3rwind in https://github.com/beetbox/beets/pull/5832#pullrequestreview-2984843935_

Start by tracing the duplicates plugin's command-line option handling and how the --delete and --tag actions are executed. Add focused tests for conflicting action options and define completion by ensuring those combinations are handled consistently without invalid follow-up operations.

Written by the indexing model from the issue text.