cmd/go: fix 'go get vanity/repo/...' in clean GOPATH

The pattern was only working if the checkout had
already been done, but the code was trying to make
it work even the first time. Test and fix.

Fixes issue 8335.

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews, iant
https://codereview.appspot.com/146310043
cmd/go: re-resolve and check vcs roots during go get -u

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 issue 8548.

LGTM=iant
R=golang-codereviews, iant
CC=adg, golang-codereviews, n13m3y3r, r
https://codereview.appspot.com/147170043