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/build.gradle

@@ -1,8 +1,21 @@
-subprojects { subproj ->
-    if (subproj.buildFile.exists()) {
-        apply plugin: 'apollo-plugin'
+gradle.projectsEvaluated {
+    configure(subprojects.findAll { it.buildFile.exists() }) { subproj ->
         apply from: "$rootDir/gradle/kotlin.gradle"
 
+        sourceSets {
+            main {
+                kotlin {
+                    srcDirs += "src"
+                }
+            }
+
+            test {
+                kotlin {
+                    srcDirs += "test"
+                }
+            }
+        }
+
         test {
             useJUnitPlatform()
         }
@@ -14,8 +27,7 @@ subprojects { subproj ->
         }
 
         dependencies {
-            test.useJUnitPlatform()
             implementation group: 'com.google.guava', name: 'guava', version: guavaVersion
         }
     }
-}
+}