Allow follows in top-level inputs and validate flake inputs

[sandydoo]

Fixes #1049.

There are two commits to this:

1. Removing null attributes when serializing the flake input struct. This fixes Can't follow nixpkgs version provided by custom flake #1049.
2. Validating that only one of url and follows are set.

2 is a little controversial because Nix actually allows setting both url and follows at the same time. The following example will not trigger an error in Nix:

{
  nixpkgs = {
    url = "...";
    follows = "other-input/nixpkgs";
  };
}

Nix seems to implicitly prioritise follows over url, which could be quite confusing. Our choice is whether to do better than Nix in this situation and throw a descriptive error, or allow it through and let Nix decide what's best.