refactor/importgraph: fix a race condition.
refactor/importgraph: fix a race condition.

The main goroutine wasn't waiting for the two closed channels to drain.

Moral: with concurrency, never invent.  D'oh.

LGTM=sameer
R=sameer
CC=golang-codereviews
https://golang.org/cl/178090043

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