Restore 'KotlinGradleSubplugin' as marker interface.

This avoids build failure in 'buildSrc' module with 'kotlin-dsl' applied
with 'ClassNotFound' exception.

^KT-51378 Fixed

libraries/tools/kotlin-gradle-plugin-api/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/KotlinGradleSubplugin.kt

@@ -116,3 +116,10 @@ interface KotlinCompilerPluginSupportPlugin : Plugin<Project> {
 open class SubpluginArtifact(val groupId: String, val artifactId: String, val version: String? = null)
 
 class JetBrainsSubpluginArtifact(artifactId: String) : SubpluginArtifact(groupId = "org.jetbrains.kotlin", artifactId = artifactId)
+
+/**
+ * Marker interface left here for backward compatibility with older plugin versions.
+ * Remove once minimal supported Gradle version will use Kotlin 1.7+.
+ */
+@Deprecated(level = DeprecationLevel.HIDDEN, message = "")
+interface KotlinGradleSubplugin

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/SubpluginsIT.kt

@@ -11,7 +11,6 @@ import org.jetbrains.kotlin.gradle.testbase.*
 import org.jetbrains.kotlin.gradle.util.checkBytecodeContains
 import org.jetbrains.kotlin.test.util.JUnit4Assertions.assertTrue
 import org.junit.jupiter.api.DisplayName
-import kotlin.io.path.readText
 
 @DisplayName("Other plugins tests")
 @OtherGradlePluginTests
@@ -212,4 +211,36 @@ class SubpuginsIT : KGPBaseTest() {
             }
         }
     }
+
+    @DisplayName("KT-51378: Using 'kotlin-dsl' with latest plugin version in buildSrc module")
+    @GradleTestVersions(
+        minVersion = TestVersions.Gradle.G_6_8 // Gradle usage of sam-with-receivers subplugin was added in this version
+    )
+    @GradleTest
+    fun testBuildSrcKotlinDSL(gradleVersion: GradleVersion) {
+        project("buildSrcUsingKotlinCompilationAndKotlinPlugin", gradleVersion) {
+            subProject("buildSrc").buildGradleKts.modify {
+                //language=kts
+                """
+                ${it.substringBefore("}")}
+                }
+                
+                buildscript {
+                    val kotlin_version: String by extra
+                    repositories {
+                        mavenLocal()
+                    }
+                    
+                    dependencies {
+                        classpath("org.jetbrains.kotlin:kotlin-gradle-plugin:${'$'}kotlin_version")
+                    }
+                }
+                
+                ${it.substringAfter("}")}
+                """.trimIndent()
+            }
+
+            build("assemble")
+        }
+    }
 }