internal/fsys: follow root symlink in fsys.Walk

internal/fsys: follow root symlink in fsys.Walk

If fsys.Walk is called with a root directory that is a symlink, follow
the symlink when doing the walk. This allows for users setting their
current directory to a symlink to a module.

Fixes #50807

Change-Id: Ie65a7cb804b87dea632ea6c758c20adcfa62fcd4
Reviewed-on: https://go-review.googlesource.com/c/go/+/448360
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>