Refactor method signature verifier.

- Extract `could_contain_kw_args?` method that does not consider
  what the last arg is but does the rest of the `has_kw_args_in?`
  logic.
- Simplify `has_kw_args_in?` so that it doesn’t take the confusing
  `kw_check` arg that can make it lie a bit.
- Leverage a base class implementation for `split_args`.