FileState/Copy task support for symlinks

The Copy task currently uses a `FileState` which is a wrapper around a `WIN32_FILE_ATTRIBUTE_DATA`. That allows it to populate timestamps + file size with a single syscall, but if the input is a symlink, the data will be for the symlink, not its target. That means that additional work may be done in a `` invocation, because the timestamp of the copy destination from last time will match the symlink target, not the symlink itself.

This is not a terrible problem, because Copy doesn't behave incrementally based on timestamps (like target execution). It uses timestamp + file size as a proxy for identicality. If it looks at a symlink, the timestamp and file size will differ, so the copy will happen. Since the copy destination's timestamp will match the source file's after the copy, this won't cause a rebuild cascade, because anything downstream that consumes the copied file will see the correct timestamp.

It's tempting to say that we could just call [`GetFileInformationByHandle`](https://msdn.microsoft.com/en-us/library/windows/desktop/aa364952%28v=vs.85%29.aspx) instead of [`GetFileAttributesEx`](https://msdn.microsoft.com/en-us/library/windows/desktop/aa364946%28v=vs.85%29.aspx), but `BY_HANDLE_FILE_INFORMATION` doesn't have a size field, so that doesn't get us the full set of data we'd want.