[Build] Switch Gradle plugins compilation to BTA

From the next bootstrap update, our Gradle plugins will be compiled via the Build Tools API using a fixated compiler version. Currently, it's `2.0.20-RC`. It allows to untie language version support in the compiler from our plugins. We may still want to use some old language versions in order to support old Gradle versions, however we do not want to create an additional constraint for language version support in Kotlin compiler.
^KT-69907 Fixed

build.gradle.kts

@@ -9,10 +9,12 @@ buildscript {
     // see also configureCompilerClasspath@
     val bootstrapCompilerClasspath by configurations.creating
     val bootstrapBuildToolsApiClasspath by configurations.creating
+    val gradlePluginsBuildToolsApiClasspath by configurations.creating
 
     dependencies {
         bootstrapCompilerClasspath(kotlin("compiler-embeddable", bootstrapKotlinVersion))
         bootstrapBuildToolsApiClasspath(kotlin("build-tools-impl", bootstrapKotlinVersion))
+        gradlePluginsBuildToolsApiClasspath(kotlin("build-tools-impl", libs.versions.kotlin.`for`.gradle.plugins.compilation.get()))
 
         classpath("org.jetbrains.kotlin:kotlin-build-gradle-plugin:${kotlinBuildProperties.buildGradlePluginVersion}")
     }

gradle/libs.versions.toml

@@ -38,6 +38,7 @@ compose-stable = "1.7.0-alpha08"
 compose-snapshot-id = "12141312"
 compose-snapshot-version = "1.8.0-SNAPSHOT"
 ant = "1.10.14"
+kotlin-for-gradle-plugins-compilation = "2.0.20-RC"
 # Wasm specific
 wasmedge = "0.14.0"
 

repo/gradle-build-conventions/buildsrc-compat/src/main/kotlin/GradleCommon.kt

@@ -587,6 +587,14 @@ fun Project.configureKotlinCompileTasksGradleCompatibility() {
             )
         }
     }
+    project.extra["kotlin.compiler.runViaBuildToolsApi"] = true
+    afterEvaluate {
+        val gradlePluginsBuildToolsApiClasspath by rootProject.buildscript.configurations
+        configurations.findByName("kotlinBuildToolsApiClasspath")?.let {
+            it.dependencies.clear()
+            dependencies.add(it.name, files(gradlePluginsBuildToolsApiClasspath))
+        }
+    }
 }
 
 // Will allow combining outputs of multiple SourceSets