RuleSelector implemented PartialOrd & Ord because ruff::flake8_to_ruff
was using RuleSelector within a BTreeSet (which requires contained
elements to implement Ord). There however is no inherent order to
rule selectors, so PartialOrd & Ord should not be implemented.
This commit changes BTreeSet<RuleSelector> to HashSet<RuleSelector>
and adds an explicit sort calls based on the serialized strings,
letting us drop the PartialOrd & Ord impls in favor of a Hash impl.
0b7fa64481