go.tools/cmd/godex: make relative package paths work
go.tools/cmd/godex: make relative package paths work

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://codereview.appspot.com/89160044

Committer: Alan Donovan <7069e9552f9d110b0a0f326b96f22f63fb99dfc1@google.com>

t-yuki/go.tools.old,t-yuki/go.tools.old,t-yuki/go.tools.old