Don't fallback to default (empty) package if the specified package doesn't exist.

Example:

```
TestedProgram server1 = new TestedProgram("chat.server");
TestedProgram client1 = new TestedProgram("chat.client");
```

Will result in executing the same class if the user haven't created `chat/server` and `chat/client` packages and created just single main class because they both will fallback to the default package.