Remove apollo-plugin Gradle plugin

Replaces the old Gradle plugin with plain Gradle buildscripts.  Also cleans-up
the current Gradle extensions and re-adds detekt, along with JUnit 5 support to
all modules.

game/plugin/api/build.gradle

@@ -1,4 +1,16 @@
-plugin {
-    name = "Apollo Plugin API"
-    description = "Helpers and API for common plugin usecases"
-}
+apply plugin: 'org.jetbrains.kotlin.jvm'
+apply plugin: 'java'
+description = 'Helpers and API for common plugin usecases'
+
+test {
+    useJUnitPlatform()
+}
+
+dependencies {
+    implementation project(':game')
+    implementation project(':cache')
+    implementation project(':net')
+    implementation project(':util')
+
+    testImplementation project(':game:plugin-testing')
+}