Issues with Gradle Kotlin DSL

Thanks for creating this extension!

**Describe the bug**
I want to use Kotlin DSL to configure Gradle build for a project which uses Java. However, the extension produces a lot of errors.

**To Reproduce**
Here is my `build.gradle.kts` file:
```
plugins {
java
application
}

repositories {
jcenter()
}

dependencies {
implementation("junit:junit:4.13")
testImplementation("junit:junit:4.13")
}

configurations {
implementation {
resolutionStrategy.failOnVersionConflict()
}
}

sourceSets {
main {
java.setSrcDirs(listOf("src"))
}
test {
java.setSrcDirs(listOf("src"))
}
}

java {
sourceCompatibility = JavaVersion.VERSION_11
targetCompatibility = JavaVersion.VERSION_11
}

application {
mainClassName = "app.App"
}

val test by tasks.getting(Test::class) {
useJUnitPlatform()
}
```
Here are the errors:

**Extension version**
0.2.11

Start by reproducing the reported errors with the provided build.gradle.kts configuration and extension version 0.2.11. Trace how the extension handles Gradle Kotlin DSL and compare its behavior with the Java and application plugins; done means the configuration no longer produces the reported errors.

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