Rework the hermeticity heuristic

[guibou]

In #82 we discussed the fact that the hermeticity heuristic implemented in nixpkgs_package is too complicated.

The current implementation parses the output of nix-build -vv and tries to detect references to files which are not listed in nix_file_deps. This parsing is fragile and complicated.

@mboes proposed during the #82 discussion to replace this code by a simple copy (symlink) of all the files listed in nix_file_deps in another temporary workspace in which the nix-build will be executed. This solution does not track dependencies on absolute path, but it way simpler.