go.tools/importer: fix classic closing-over-induction-variable gotcha.
go.tools/importer: fix classic closing-over-induction-variable gotcha.

Now we actually prefetch all n packages, instead of
prefetching the last one n times.  This yields a further 22%
improvement, which is more like what I was hoping for.

It makes me so sad that Go reproduced one of the best-known
mistakes of JavaScript.  D'oh!

R=crawshaw
CC=golang-dev
https://golang.org/cl/13379046

josharian/tools,syreclabs/go-tools,jdhenke/tools,josharian/tools,argandas/tools,runcom/tools,maddyonline/tools,maddyonline/tools,syreclabs/go-tools,cw2018/tools,golang/tools,springning/tools,nicot/tools,springning/tools,glasser/golang-tools,nicot/tools,cmars/tools,syreclabs/go-tools,4ad/tools,magiccao/tools,dmitris/tools,runcom/tools,argandas/tools,magiccao/tools,cmars/tools,matryer/tools,golang/tools,syreclabs/go-tools,cmars/tools,glasser/golang-tools,syreclabs/go-tools,sshetty10/tools,glasser/golang-tools,cmars/tools,maddyonline/tools,syreclabs/go-tools,matryer/tools,golang/tools,runcom/tools,cw2018/tools,jdhenke/tools,matryer/tools,matryer/tools,sshetty10/tools,argandas/tools,sshetty10/tools,nicot/tools,dmitris/tools,dmitris/tools,jdhenke/tools,springning/tools,sshetty10/tools,magiccao/tools,dmitris/tools,josharian/tools,golang/tools,4ad/tools,jdhenke/tools,cw2018/tools,runcom/tools,cw2018/tools,nicot/tools,josharian/tools,glasser/golang-tools,4ad/tools,golang/tools,maddyonline/tools,springning/tools,argandas/tools,magiccao/tools