Yeah, did it to remove the necessity for source files that should be bound to mailpile.plugin.[name] to be directly in the root folder of the plugin. 

This is fine for internal plugins, but for external ones it causes some problems, especially if they have some more source files with their own module hierarchy. Currently all plugins consist of only one python file and that is always named like the plugin itself. So before introducing some sophisticated "sources_root" attribute to the manifest I made this change.