Supports unix-style binary_has_prefix paths in windows binaries

Some windows libraries (like Qt5) encode their hardcoded paths with unix-style path separators. This pull request identifies such paths, allowing the library to function when relocated by `conda install`.

Also changed: a warning is issued if the prefix cannot be found in binary files tagged for replacement, and the file is not added to `has_prefix` metadata since replacement would not succeed on install.