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 agree about not changing the default placeholder. The placeholders defined in `build.py` are implementation details, and do not need to be advertised. They might as well be `/conda_build_prefix` or `/this/path/will/be/replaced/at/install`.

This pull request could be modified to continue using the default placeholder, and use a different placeholder when necessary. I recall when I first got started with conda asking on the mailing list why /opt/anaconda1anaconda2anaconda3 appeared in hashbangs on my windows machine.