Gradle task spotlessInternalRegisterDependencies configured eagerly

When running `./gradlew help --scan` the Build Scan reports task `spotlessInternalRegisterDependencies` is configured. Gradle's task configuration avoidance feature should be used so it's only configured when needed.

Affects spotless on main branch. The problem was introduced by https://github.com/diffplug/spotless/pull/1039, thanks to `get()` call here:

https://github.com/diffplug/spotless/blob/f4ee554c4600360a678c5faa2e447df919d536f2/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/SpotlessExtension.java#L52