In hydra-evaluator, reuse an SVN working copy between runs (similar to
what we do with Git and other input types). This reduces network
traffic in the common case.
Also, don't use nix-prefetch-svn. It doesn't do anything useful.
The underscores are ugly and the .pl extension is an implementation
detail that shouldn't be visible to the outside.
Also, get rid of the *.in files. It's not really necessary to
generate them. And I was always modifying the wrong file.
