Fix file packager on Windows.

Fixes file packager to generate a virtual filesystem with '/' as path separators when the host is Windows.

The recent file packager fixes had an unfortunate side effect of making Windows builds generate VFS filenames using backslashes as directory delimiters, which would get treated as an escape character in JS, and loading the generated .html files would fail with an invalid symbol error. This restores browser.test_preload_file to work on Windows.