Remove comparator.hh and switch to <=> in a bunch of places

Known behavior changes:

- `MemorySourceAccessor`'s comparison operators no longer forget to
  compare the `SourceAccessor` base class.

Progress on #10832

What remains for that issue is hopefully much easier!

src/libutil/args.cc

@@ -57,8 +57,7 @@ void Completions::add(std::string completion, std::string description)
     });
 }
 
-bool Completion::operator<(const Completion & other) const
-{ return completion < other.completion || (completion == other.completion && description < other.description); }
+auto Completion::operator<=>(const Completion & other) const noexcept = default;
 
 std::string completionMarker = "___COMPLETE___";