scrubs all build prefixes

This pull request scrubs unix-style build prefixes in windows conda packages.

It also explicitly lists all files containing a build prefix placeholder, and does not use the `/opt/anaconda1anaconda2anaconda3` default placeholder. The handling of `/opt/anaconda1anaconda2anaconda3` is somewhat anomolous, since it is not explicitly recorded in `has_prefix`, and it is a unix-style path that gets replaced with a windows style path at installation time.

I don't see any backward compatibility consequences for not producing packages with the old default placholder.
I think I'm OK with changing what conda-build uses as its default placeholder (though I don't necessarily think it needs to be done), but I think we should only do so via what is replaced in the file after the build. We already have one magic placeholder that works automatically.