cmd/go/internal/imports: resolve symlinks in ScanDir

cmd/go/internal/imports: resolve symlinks in ScanDir

We were using the mode reported by ReadDir to decide whether each
entry is a file, but in the case of symlinks that isn't sufficient: a
symlink could point to either a file or a directory, and if it is a
file we should treat it as such.

Fixes #28107

Change-Id: Icf6e495dce427a7b1124c9cc9f085e40a215c169
Reviewed-on: https://go-review.googlesource.com/c/141097
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>