cmd/api: fix tool for recent go/build change

Asking about runtime/cgo when CgoEnabled=false now correctly
returns an error from build.Import (specifically, NoGoError), because
there are no buildable Go files in that directory.

The API tool was depending on it returning a package with no Go
files instead. Correct that assumption.

Fixes all.bash on local machines.
(Dashboard appears not to be running the api tool at all.)

Update issue 6124

TBR=golang-dev
CC=golang-dev
https://codereview.appspot.com/13385046