make 'make package' package the helper addon

`make package` fails with an error:

```
11-05 14:50 > make package
cd addon-sdk && . bin/activate && cd ../addon && cfx xpi
Welcome to the Add-on SDK. Run 'cfx docs' for assistance.
package.json does not have a 'main' entry.
make: *** [package] Error 1
```

I think that's because it's trying to `cfx xpi` in the *addon/* directory, which is now just a package of modules, instead of the *helper/* directory, which contains the *main.js* and *package.json* files for the helper addon.

If so, then this is the right fix, although I haven't tested resultant addon package.

@ochameau does this look correct to you?