Build: introduce javaPluginConvention extension on project

`ExtensionAware.the<T>()` introduced in gradle 4.7 made existing calls `the<JavaPluginConvention>()` invalid (on wrong receiver)

libraries/tools/kotlinp/build.gradle.kts

@@ -32,7 +32,7 @@ val shadowJar by task<ShadowJar> {
     classifier = "shadow"
     version = null
     configurations = listOf(shadows)
-    from(the<JavaPluginConvention>().sourceSets.getByName("main").output)
+    from(javaPluginConvention().sourceSets.getByName("main").output)
     manifest {
         attributes["Main-Class"] = "org.jetbrains.kotlin.kotlinp.Main"
     }