Allows most existing configs to continue building, now with warnings
instead of assertions when the old `plugins.*.*Package` options are
used.
An assertion will still occur if there is a merge conflict, e.g:
`plugins.a.fooPackage = null` and `plugins.b.fooPackage = pkgs.foo`.
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>
