Compile error when a function parameter name is same with package name

See example:
```go
package packet

func GoPacketToBytes(shieldID int, packet Packet) ([]byte, error) { // gopy bug
buf := bytes.NewBuffer([]byte{})
w := protocol.NewWriter(buf, int32(shieldID))
packet.Marshal(w)
return buf.Bytes(), nil
}
```
Use `gopy build` it will generate Go file which includes:
```go
packet.GoPacketToBytes( ... )
```
which causes compile problem.
Rename parameter `packet` could solve it, but it's better to fix this problem since the error log is weird to greenhands.