If you do 'go get -u rsc.io/pdf' and then rsc.io/pdf's redirect
changes to point somewhere else, after this CL a later
'go get -u rsc.io/pdf' will tell you that.

Fixes #8548.

LGTM=iant
R=golang-codereviews, iant
CC=adg, golang-codereviews, n13m3y3r, r
https://golang.org/cl/147170043