Fix localized files with same name

[ryohey]

• Refactor the code generating variant groups at e24c0e2 ce27af0. These do not change any output.
• Fix localized files with same name at 66a2893. This will add localized files into the variant group that matches the file path with extension first. We use the variant group matched the file name without extension secondary for the case such as storyboard and strings.

close #122

[yonaskolb]

Nice one! Thanks @ryohey!

[yonaskolb]

By the way @ryohey I'm not sure if you're in the slack group, but you're welcome to join. We discuss XcodeGen and the projects from https://github.com/xcodeswift
http://xcodeswift.herokuapp.com

Anyone else who is reading this is free to join too, it's an open community 👍