Small refactoring in output.py.

    * remove duplication in handling `from_first`
    * `with_from_import` and `with_straight_import` don't need
      to know or write to the "global" `section_output`
       - remove `section_output` argument
       - remove `section_output.copy()`
       - functions return their own output without knowing about
         the "global" output
    * `sort_ignore_case` already passed in to `_with_from_imports` in
      `config`. Just use it from there.