Build: Introduce Project extension properties for source sets

prepare/formatter/build.gradle.kts

@@ -10,7 +10,7 @@ runtimeJar {
     dependsOn(":idea:formatter:classes")
     project(":idea:formatter").let { p ->
         p.pluginManager.withPlugin("java") {
-            from(p.javaPluginConvention().sourceSets.getByName("main").output)
+            from(p.mainSourceSet.output)
         }
     }
     from(fileTree("$rootDir/idea/formatter")) { include("src/**") } // Eclipse formatter sources navigation depends on this