Link.Plists never contained more than one Plist, and sometimes none.
Passing around the Plist being worked on is straightforward and makes
the data flow easier to follow.

Change-Id: I79cb30cb2bd3d319fdbb1dfa5d35b27fcb748e5c
Reviewed-on: https://go-review.googlesource.com/37169
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>