Spotless for imports

Currently, our import ordering in spoon is a bit messy. This leads to an unpleasant time with IntelliJ trying to sort the imports in a more normal order. Why don't we use spotless-maven-plugin to sort all our imports in a fixed sorting? This also helps to reduce diffs in the future for PRs. The benefit compared to checkstyle is that we can do the sorting automatic. See https://github.com/diffplug/spotless/tree/main/plugin-maven#java for an example.