flakes: throw an error if follows-declaration for an input is invalid

[Ma27]

I recently got fairly confused why the following expression didn't have
any effect

{
  description = "Foobar";
  inputs.sops-nix = {
    url = github:mic92/sops-nix;
    inputs.nixpkgs_22_05.follows = "nixpkgs";
  };
}

until I found out that the input was called nixpkgs-22_05 (please note
the dash vs. underscore).

IMHO it's not a good idea to not throw an error in that case and
probably leave end-users rather confused, so I implemented a small check
for that which basically checks whether follows-declaration from
overrides actually have corresponding inputs in the transitive flake.

In fact this was done by accident already in our own test-suite where
the removal of a follows was apparently forgotten[1].

Since the key of the std::map that holds the overrides is a vector
and we have to find the last element of each vector (i.e. the override)
this has to be done with a for loop in O(n) complexity with n being
the total amount of overrides (which shouldn't be that large though).

Please note that this doesn't work with nested expressions, i.e.

inputs.fenix.inputs.nixpkgs.follows = "...";

which is a known problem[2].

For the expression demonstrated above, an error like this will be
thrown:

error: sops-nix has a `follows'-declaration for a non-existant input nixpkgs_22_05!

[1] 2664a21
[2] #5790

cc @edolstra @thufschmitt @Ericson2314

[Ma27]

ping @edolstra @thufschmitt would you mind taking a look? :)

[thufschmitt]

Thanks :)

A few comments, but looks good overall