pyenchant: Fix for darwin/macOS

This allows pyenchant to be installable on macOS again, which, in turn, will allow pylint
to also be installable.

  * Switches dependencies to enchant-2 (enchant-1 is not building on macOS). The existing $src (2.0.0) already has compatibility with enchant-2.
  * Improves patch hack by hijacking the $PYENCHANT_LIBRARY_PATH envvar lookup
    to explicitly specify the correct library path.