go.tools/dashboard/builder: rewrite sub-repo paths before 'go get' and 'go test'
go.tools/dashboard/builder: rewrite sub-repo paths before 'go get' and 'go test'

The dashboard still uses the old "code.google.com/p/go.*" import paths.
Rewrite them to the new "golang.org/x/*" before running 'go get' and 'go test'.

Update golang/go#9079.

LGTM=rsc
R=rsc, bradfitz
CC=golang-codereviews
https://golang.org/cl/174740043

proppy/build,golang/build,evandbrown/build,proppy/build,proppy/build,proppy/build,evandbrown/build,golang/build,evandbrown/build,golang/build,golang/build