gh-138697: Fix inferring dest from a single-dash long option in argparse (#138699)

* gh-138697: Fix inferring dest from a single-dash long option in argparse

If a short option and a single-dash long option are passed to add_argument(),
dest is now inferred from the single-dash long option.

* Make double-dash options taking priority over single-dash long options.

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Co-authored-by: Savannah Ostrowski <savannah@python.org>