dependenciesVersion is null by default in AtomicFUPluginExtension

[whyoleg]

When using gradle 6+ and adding atomicfu-gradle-plugin to classpath through settings.gradle[.kts] results in pluginVersion=null and as a result runtime dependency isn't added at all.
The problem is here:
https://github.com/Kotlin/kotlinx.atomicfu/blob/cffe6eeef32eec8a7dca0f7d414b7ae1b1306bd2/atomicfu-gradle-plugin/src/main/kotlin/kotlinx/atomicfu/plugin/gradle/AtomicFUGradlePlugin.kt#L30
There are at least 2 workarounds:

• set version explicitly
• add atomicfu-gradle-plugin to classpath in root project
  But it will be good if it will be fixed, so all classpathes for plugins could be configured inside settings.gradle[.kts]