afterEvaluate AbstractMutationGuard$IllegalMutationException

I am getting
```
Caused by: org.gradle.api.reflect.ObjectInstantiationException: Could not create an instance of type com.diffplug.gradle.spotless.SpotlessExtensionImpl.
at at org.gradle.internal.instantiation.generator.DependencyInjectingInstantiator.doCreate(DependencyInjectingInstantiator.java:70)
at org.gradle.internal.instantiation.generator.DependencyInjectingInstantiator.newInstanceWithDisplayName(DependencyInjectingInstantiator.java:51)
at org.gradle.internal.extensibility.DefaultConvention.instantiate(DefaultConvention.java:229)
at org.gradle.internal.extensibility.DefaultConvention.create(DefaultConvention.java:150)
at org.gradle.internal.extensibility.DefaultConvention.create(DefaultConvention.java:145)
at com.diffplug.gradle.spotless.SpotlessPlugin.apply(SpotlessPlugin.java:53)
```
due to:
```
Caused by: org.gradle.api.internal.AbstractMutationGuard$IllegalMutationException: Project#afterEvaluate(Action) on project ':x:x-core' cannot be executed in the current context.
at org.gradle.api.internal.AbstractMutationGuard.createIllegalStateException(AbstractMutationGuard.java:39)
at org.gradle.api.internal.AbstractMutationGuard.assertMutationAllowed(AbstractMutationGuard.java:34)
at org.gradle.api.internal.project.DefaultProject.assertMutatingMethodAllowed(DefaultProject.java:1477)
at org.gradle.api.internal.project.DefaultProject.afterEvaluate(DefaultProject.java:1054)
at com.diffplug.gradle.spotless.SpotlessExtensionImpl.(SpotlessExtensionImpl.java:42)
```

Following some chat in the Gradle Slack chat:

> looks like a bug in the plugin implementation, so I'd raise a bug
`afterEvalute {}` should be avoided wherever possible, and I don't think it's needed here
https://github.com/diffplug/spotless/blob/1108c14f83d8e96bc86f923acebfedb5e4a43917/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/SpotlessExtensionImpl.java#L42
> you could try setting `enforceCheck = false` and setting up the task dependency in a more idiomatic way

plugin-gradle/src/main/java/com/diffplug/gradle/spotless/SpotlessExtensionImpl.java の42行目付近から始め、afterEvaluate アクションがどのように登録されているかを確認してください。報告された Gradle の失敗を再現し、その後、AbstractMutationGuard 例外なしでプラグインを適用できること、および既存の enforceCheck の動作が引き続き機能することを確認してください。

索引モデルが issue の本文から書いたものです。