Creating custom Gradle PlugIn, but cannot compile it: "Unresolved reference: sourceSets"

I am writing a custom Gradle plugin to configure our common Kotlin build settings, which includes creating an integrationTest sourceSet. I created a simple Kotlin project to ensure that I had everything correct in a build.gradle.kts file before translating it into the custom plugin project. I am getting a compilation error in the plugin:

"Unresolved reference: sourceSets"

I think that the issue may be because I am trying to access the sourceSet in the plugin in an incorrect manner. I saw some references online about using

the<SourceSetContainer>()

rather than how I am accessing the sets, below. However, I cannot seem to get this to work in the custom plugin...compilation error.

I'd appreciate your help!

class CscKotlinPlugin: Plugin<Project> {

    private lateinit var javaPluginExtension: JavaPluginExtension

    override fun apply(project: Project) {

        with(project) {
            javaPluginExtension = project.extensions.getByType(JavaPluginExtension::class.java)
            configureBuild()
            . . .

    private fun Project.configureBuild() {
        val implementation = configurations.getByName("implementation")
        val integrationTestImplementation =
            configurations.getByName("integrationTestImplementation").also { it.extendsFrom(implementation) }

        val runtimeOnly = configurations.getByName("runtimeOnly")
        val integrationTestRuntimeOnly =
            configurations.getByName("integrationTestRuntimeOnly").also { it.extendsFrom(runtimeOnly) }

        with(javaPluginExtension) {
            sourceCompatibility = JavaVersion.VERSION_11
            targetCompatibility = JavaVersion.VERSION_11
            withJavadocJar()
            withSourcesJar()

            with(sourceSets) {
                val main = getByName("main").let { main ->

                    create("integrationTest") { sourceSet ->
                        sourceSet.compileClasspath += main.output
                        sourceSet.runtimeClasspath += main.output

                        sourceSet.resources.srcDir("src/integrationTest/resources")
                        sourceSet.java { sourceDirectorySet ->
                            sourceDirectorySet.srcDir("src/integrationTest/java")
                            sourceDirectorySet.srcDir("src/integrationTest/kotlin")
                        }
                    }
                }

                val test = findByName("test")
            }
        }
    }