cmd/link: linking .a file not in package main succeeds

[ianlancetaylor]

foo.go:

package p
import "fmt"
func main() {
    fmt.Println("Hello, world");
}

go tool compile foo.go
go tool link foo.go
/home/iant/go/pkg/tool/linux_amd64/link: foo.o: not package main (package p)

go tool compile -pack foo.go
go tool link foo.a
./a.out
Hello, world

The link should not succeed in the -pack case. It should get the same error as the non-pack case.