Update to Kotlin 1.60 and Gradle 4.9

Removes the dependency on legacy script compilation and now relies on script
discovery.  In addition, the Gradle build scripts were refactored and updated
to be compatible with Gradle 5.0 and make use of the new java-library
configurations.

buildSrc/src/main/groovy/org/apollo/build/plugin/ApolloPluginExtension.groovy

@@ -47,30 +47,19 @@ class ApolloPluginExtension {
      * and its scripts.
      */
     def init() {
-        def gameProject = project.findProject(":game")
-        def pluginTestingProject = project.findProject(':game:plugin-testing')
-
         project.plugins.apply('kotlin')
         project.dependencies {
-            def transitiveGameDeps = gameProject.configurations["compile"].dependencies
-            def gameSources = gameProject.sourceSets.main
-
-            transitiveGameDeps.each { dependency ->
-                compile dependency
-            }
-
-            compile gameSources.output
-            testCompile pluginTestingProject
+            implementation project.findProject(':game')
+            implementation project.findProject(':cache')
+            implementation project.findProject(':net')
+            implementation project.findProject(':util')
+            testImplementation project.findProject(':game:plugin-testing')
         }
 
         project.sourceSets {
             main.kotlin.srcDirs += this.srcDir
             test.kotlin.srcDirs += this.testDir
         }
-
-        project.tasks["compileKotlin"].kotlinOptions.freeCompilerArgs += [
-                "-script-templates", "org.apollo.game.plugin.kotlin.KotlinPluginScript"
-        ]
     }
 
     def getDependencies() {