cd rundir

! go run x.go sub/sub.go
stderr 'named files must all be in one directory; have . and sub'
! go run sub/sub.go x.go
stderr 'named files must all be in one directory; have sub and .'

cd ../
go run rundir/foo.go ./rundir/bar.go
stderr 'hello world'

-- rundir/sub/sub.go --
package main
-- rundir/x.go --
package main
-- rundir/foo.go --
package main
func main() { println(msg) }
-- rundir/bar.go --
package main
const msg = "hello world"
