Define `call = callPackageWith { inherit pkgs lib helpers; }`, which can
be used to automatically pass the correct args into helpers files.
`helpers` is passed in recursively.
Based on `types.coerceTo`, which is like `types.either` but coerces the
left-hand type into the right-hand type.
`transitionType` only shows the right-hand type in its description and
also prints a warning when the left-hand type is used.
Co-authored-by: Silvan Mosberger <contact@infinisil.com>
