go.tools/cmd/godoc: search go.tools/cmd before $GOROOT/cmd
go.tools/cmd/godoc: search go.tools/cmd before $GOROOT/cmd

Fixes golang/go#6527.

R=rsc
CC=golang-dev
https://golang.org/cl/14725043

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