[suggestion]: add CtInterface.setPackage() to be used together with CtInterface.setSimpleName()

Q: is there an example, given a class, generate an interface for it?

ok, I tried to
```
CtInterface itfc = getFactory().createInterface();
itfc.setSimpleName(interfaceName);
...
clazz.addSuperInterface(itfc.getReference()); // added the new interface to an existing clazz
```

However in the output, I only get the "ExistingClass implements the NewInterface", but there is no output file for NewInterface.java itself.

so which call will write a generated CtInterface to disk file?

Thanks!