Add some way to ignore files

[thiagokokada]

There are a few cases that the checks in nixpkgs-vet doesn't make sense. For example in PR #369151, the error is in nixos-option.nix file that has a good reason to import <nixpkgs/nixos> { }, but nixpkgs-vet fails with:

pkgs/by-name/ni/nixos-option: File nixos-option.nix at line 2 contains the nix search path expression "<nixpkgs/nixos>" which may point outside the directory of that package.

Another example is in nixos-rebuild-ng, that has a Python's template file with Nix code that nixpkgs-vet complained when I named it repl.template.nix, so I had to rename to repl.nix.template instead. Not ideal because the file lost its syntax highlight and I needed to add a Vim modeline to fix it (but of course this doesn't work in every text editor).

In those 2 cases, it is more than valid to just ignore whatever nixpkgs-vet is complaining. Would also be nice to have the possibility to ignore particular errors (i.e.: #99), but ignoring whole files would be a good first step.

[philiptaron]

Agree in principle that a way to avoid specific checks on specific files is worth it.