nixpkgs-vet should error if a package accepts an input that is never used

[Pandapip1]

This helps catch inputs that were brought in for testing and never removed.

E.G:

When I need to inspect what the output of a build is, I often set preInstall to run tree and exit 1. So:

{
  ...,
  tree,
  ...
}:

... # Doesn't use tree at all

should be an error

[pbsds]

Please elaborate why you think this should be an error

[Pandapip1]

Well, it indicates that there is almost certainly a coding error. In all the scenarios where I think this could happen except for the first, it warrants blocking the PR.

1. A dependency was added for testing and was then removed without removing it from the function inputs. This is a packaging error and should be fixed, but is the only situation in which I could that a warning would be more appropriate as it's not likely to cause confusion for users, nor does it indicate that it was mis-packaged.
2. The packager thought they had added the dependency to the {nativeBuild,build,nativeCheck,check,...}Inputs, but it turns out they forgot to. This should be an error because if the packager intended to add the package but it wasn't added, and the package still builds, then the dependency is probably an Automagic dependency and there will almost certainly be missing functionality.
3. A flag was added for testing and was then removed from the body. The flag should be removed or commented out respectively in order to not confuse users into thinking that the flag will work. If I were reviewing a PR and there was a flag that wasn't used at all, I personally would block on that.
4. A flag was added with the intent to add its functionality, but either the packager forgot about it or hasn't gotten around to it yet. Either way, the logic in (3) applies.

If you can think of any other reason this might happen, LMK.