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://codereview.appspot.com/81990043

karmakaze/go.tools,chenjianlong/go.tools,chenjianlong/go.tools,karmakaze/go.tools,karmakaze/go.tools,chenjianlong/go.tools,karmakaze/go.tools,karmakaze/go.tools