[release-branch.go1.12] cmd/go/internal/imports: use the full path to resolve symlinks

info.Name returns a name relative to the directory, so we need to
prefix that directory in the Stat call.

(This was missed in CL 141097 due to the fact that the test only
happened to check symlinks in the current directory.)

This allows the misc/ tests to work in module mode on platforms that
support symlinks.

Updates #30228
Updates #28107
Fixes #31763

Change-Id: Ie31836382df0cbd7d203b7a8b637c4743d68b6f3
Reviewed-on: https://go-review.googlesource.com/c/163517


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/175441


Reviewed-by: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>