[Nix-dev] Auto package update script v3

[Tobias Hunger]

Hi Marc!

> sorry that I didn't review your code earlier.

No problem at all, I enjoyed the playing:-)

> It adds a watchfile to perl.

I talked with niksnut on IRC yesterday night about this and he raised
pretty much the same issues.

I started on version 4 yesterday with changes the following things:

* Moves all the attributes I evaluate into the meta attribute set.

* Gets rid of the watchfiles. It will extract a watchfile expression from
  the meta data of the derivation instead, put that into a temporary file
  and run uscan on that file.

  It looks a bit like what you suggested, but adds the "version=3" line
  itself. So that one needs not clutter up the .nix-file.

* Makes the (optional) url Mangling safer. Url Mangling can be used to e.g.
  turn the URL reported by uscan into the mirror syntax used in nix
  derivations. I used this in the xorg package quite a lot.

> Is uscan already in nixpkgs?

I don't think so, but it should be straight forward to add if my patch
gets accepted.

Best Regards,
Tobias