let update.py generates generated.nix as an overlay.
This should help maintaining vim-plugins outside of nixpkgs, as one can
use update.py to generate an out-of-tree vimPlugins overlay.
In response to @timokau's review here are a couple changes:
- Decrease the fragility of the replacement code by normalizing
whitespace on each line.
- Throw an error when plugins are renamed rather than silently aliasing
to the new name.
