Aliases aren't preserved in generated interfaces

This has come up when different files in the same package import different packages that share a name.

Go has no problem with this:

```go
import (
	"fmt"
	abc "fmt"
	bcd "fmt"
)

func main() {
	fmt.Println("A")
	abc.Println("B")
	bcd.Println("C")
}
```

So interfacegen should aim to preserve aliases, otherwise it will generate broken interfaces that don't compile.