fs: remove os.FileInfo from fs.ExtendedFileInfo

Only the `Sys()` value from os.FileInfo is kept as field `sys` to
support Windows. The os.FileInfo removal ensures that for values like
`ModTime` that existed in both data structures there's no more confusion
which value is actually used.

internal/fs/stat_windows.go

@@ -18,8 +18,11 @@ func extendedStat(fi os.FileInfo) *ExtendedFileInfo {
 	}
 
 	extFI := ExtendedFileInfo{
-		FileInfo: fi,
-		Size:     int64(s.FileSizeLow) | (int64(s.FileSizeHigh) << 32),
+		Name: fi.Name(),
+		Mode: fi.Mode(),
+
+		Size: int64(s.FileSizeLow) | (int64(s.FileSizeHigh) << 32),
+		sys:  fi.Sys(),
 	}
 
 	atime := syscall.NsecToTimespec(s.LastAccessTime.Nanoseconds())