path/filepath: rewrite walkSymlinks

Rather than try to work around Clean and Join on intermediate steps,
which can remove ".." components unexpectedly, just do everything in
walkSymlinks. Use a single loop over path components.

Fixes #23444

Change-Id: I4f15e50d0df32349cc4fd55e3d224ec9ab064379
Reviewed-on: https://go-review.googlesource.com/121676


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>