Consider reducing the transitive dependencies of this plugin

[jjohannes]

It is not really the fault of this plugin, but the dependencies to various Maven artifacts bring in a ton of additional Jars transitively on the Gradle plugin classpath. It is difficult to maintain a clean Gradle plugin classpath by this and adding dependency verification data for all those Jars is challenging. Adding this plugin to a build (which successfully verified all its dependencies before) gives: 59 artifacts failed verification

There are two things I can think off to improve the situation:

• Add excludes for everything that is not needed in the common case (e.g. listenablefuture-9999.0-empty-to-avoid-conflict-with-guava.jar is clearly not needed)
• Shadow dependencies in this plugin so that they become part of the artifact. This repository itself should then have a good dependency verification setup so that we make sure what we re-package is verified.

Doing something here, may help with issues like #65.

(Maybe updating to Maven 4.x as baseline also reduces the amount of transitives (?))