apiCheck isn't called for the Gradle plugin

[ALikhachev]

4d77999 introduced BCV for the Gradle plugin.
Also, it introduced the workaround

tasks.register("apiDump") {
    it.dependsOn(gradle.includedBuild("plugin").task(":apiDump"))
}

however, currently, ./gradlew apiCheck does not check the API dump of the plugin, and it is indeed outdated
Perhaps, there should be added a similar workaround for apiCheck.