Turbopack: normalize distDir separators (#86697)
Closes #86652
Closes PACK-6023
Previously, `dist_dir` contained backslashes on Windows which broke various things (among them, relative paths for the externals symlinks):
https://github.com/vercel/next.js/blob/6eed9a05450f2c179b951c97350e305973a09e6f/crates/next-api/src/project.rs#L765
