[Gradle] KotlinMultiplatformSourceSetConventions: Do not configure source sets in 'lazy' context

As source sets are not lazy. This avoids unnecessary exceptions
in user buildscripts.

^KT-60842 Verification Pending

libraries/tools/kotlin-gradle-plugin/src/common/kotlin/org/jetbrains/kotlin/gradle/dsl/KotlinMultiplatformSourceSetConventions.kt

@@ -55,17 +55,15 @@ interface KotlinMultiplatformSourceSetConventions {
 
     operator fun NamedDomainObjectProvider<KotlinSourceSet>.invoke(
         configure: KotlinSourceSet.() -> Unit,
-    ) = configure(configure)
+    ): Unit = get().run(configure)
 
     fun NamedDomainObjectProvider<KotlinSourceSet>.dependencies(
         handler: KotlinDependencyHandler.() -> Unit,
-    ) = get().dependencies(handler)
+    ): Unit = get().dependencies(handler)
 
     fun NamedDomainObjectProvider<KotlinSourceSet>.languageSettings(
         configure: LanguageSettingsBuilder.() -> Unit,
-    ) = configure { sourceSet ->
-        sourceSet.languageSettings(configure)
-    }
+    ): Unit = this { languageSettings(configure) }
 }
 
 /* Implementation */

libraries/tools/kotlin-gradle-plugin/src/functionalTest/kotlin/org/jetbrains/kotlin/gradle/unitTests/KotlinMultiplatformSourceSetConventionsTest.kt

@@ -51,6 +51,23 @@ class KotlinMultiplatformSourceSetConventionsTest {
         }
     }
 
+    @Test
+    fun `test - invoke - allows creating new source set in closure`() {
+        val project = buildProjectWithMPP()
+        project.multiplatformExtension.apply {
+            sourceSets.jvmMain {
+                /*
+                When done wrong, expect:
+                org.gradle.api.internal.AbstractMutationGuard$IllegalMutationException:
+                    NamedDomainObjectContainer#create(String) on KotlinSourceSet container cannot be executed in the current context
+                 */
+                dependsOn(sourceSets.create("foo"))
+            }
+
+            assertEquals(setOf("foo"), sourceSets.jvmMain.get().dependsOn.map { it.name }.toSet())
+        }
+    }
+
     @Test
     fun `test - languageSettings`() {
         val project = buildProjectWithMPP()