Whenever I try to create with Maven, I seem to be unable to create a Package nor Java class. However creating a blank Java project allows me to proceed as normal. However, I (had to) download the Sponge API jarfile and add it as a dependency manually instead of grabbing it directly from the repo. I seem to be missing some things (such as org.spongepowered.api.service.command.CommandService) and some are out of place (such as org.spongepowered.api.service.command.* being located at org.spongepowered.api.command.*).
Oh, my bad. I confused it with Gradle project.
You can mark any folder in project as Source folder with right click → Mark directory as → Sources root.