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.
yes, see comments in `build.py`