walkFS is used in TestWalkFS to walk over memFS FileSystem.
memFS uses slash delimited paths, so we must use path,
not filepath, package to construct path elements in walkFS.

Fixes golang/go#10517.

Change-Id: I3632472e5fb0fbaa13e5656d9ae5483e8ec332b2
Reviewed-on: https://go-review.googlesource.com/9193Reviewed-by: Robert Stepanek <robert.stepanek@gmail.com>
Reviewed-by: Nigel Tao <nigeltao@golang.org>