This reverts commit 4ff1ab5.

We need this to type options like:
services.xserver.windowManager.xmonad.extraPackages that specify functions that
take an attribute set containing packages / plugins and return a list containing
a selection of the values in this set.

The reason we need a dedicated type for this is to have the correct merge
behaviour. Without the functionTo type merging multiple function option
definitions results in an evaluation error. The functionTo type merges
definitions by returning a new function that applies the functions of all the
definitions to the given input and merges the result.

(cherry picked from commit 7ed41ff)

(cherry picked from commit 478af11)

Without this patch merging options like
services.xserver.windowManager.xmonad.extraPackages
results in the evaluation error:

  error: value is a list while a set was expected, at nixpkgs/lib/options.nix:77:23

With this patch we get the desired merging behaviour that just concatenates the
resulting package lists.

(cherry picked from commit 6e99f9f)

Co-Authored-By: Silvan Mosberger <contact@infinisil.com>

Now type checks the resulting function values and allows mkMerge and co.
Also indicates that the type check is done in the function body

Co-Authored-By: Robert Hensing <robert@roberthensing.nl>

Bring back `types.functionTo`