go.tools/cmd/godex: replicate path logic for gccgo-new importer
go.tools/cmd/godex: replicate path logic for gccgo-new importer

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://golang.org/cl/81990043

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