os: in Symlink, stat the correct target path for drive-relative targets on Windows

os: in Symlink, stat the correct target path for drive-relative targets on Windows

Previously, when the target (“old”) path passed to os.Symlink was a
“root-relative” Windows path,¹ we would erroneously prepend
destination (“new”) path when determining which path to Stat,
resulting in an invalid path which was then masked by the lack of
error propagation for the Stat call (#39183).

If the link target is a directory (rather than a file), that would
result in the symlink being created without the
SYMBOLIC_LINK_FLAG_DIRECTORY flag, which then fails in os.Open.

¹https://docs.microsoft.com/en-us/windows/win32/fileio/creating-symbolic-links

Updates #39183

Change-Id: I04f179cd2b0c44f984f34ec330acad2408aa3a20
Reviewed-on: https://go-review.googlesource.com/c/go/+/235317
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>