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 #6124

TBR=golang-dev
CC=golang-dev
https://golang.org/cl/13385046

src/cmd/api/goapi.go

@@ -145,6 +145,10 @@ func main() {
 			//   going to change w/o a language change.
 			// - We don't care about the API of commands.
 			if name != "unsafe" && !strings.HasPrefix(name, "cmd/") {
+				if name == "runtime/cgo" && !context.CgoEnabled {
+					// w.Import(name) will return nil
+					continue
+				}
 				w.export(w.Import(name))
 			}
 		}