Prevent "duplicate entry __init__.pyc"

[NiklasRosenstein]

Namespace packages all have their own __init__.pyc but usually with the same content as other packages in the same namespace. Packaging multiple packages of the same namespace into a single .egg adds all of their __init__.pyc files to the ZIP file, resulting in multiple entries of the same file.

We should check if the file being added already exists. If it does, we should then check if the file is just a namespace indicator. If it is, we can omitt it. Otherwise we should warn the user and still add multiple entries.