Make transitive AGP dependency compile-only (#168)

Otherwise, AS 3.3 and lower won’t be able to use the plugin
because it bumps up the requirement silently.
On the same token, dynamically create the Functional Test configurations
since they now need to declare the dependency explicitly.

Author: mannodermaus

plugin/android-junit5/build.gradle.kts

@@ -102,31 +102,34 @@ tasks.named("processTestResources", Copy::class.java).configure {
   }
 }
 
-// Add custom dependency configurations
+// Add custom dependency configurations for Functional Tests.
+// Different versions of the Android Gradle Plugin should be testable in the same project;
+// to do this, create a custom configuration for each version & assign the correct dependency to it.
+// At runtime, the functional tests will look up a file listing of all dependencies, making it the
+// plugin classpath for the respective test.
+data class AgpConfiguration(val version: String, val dependency: String) {
+  // Example: version = "3.2" --> configurationName = "testAgp32x"
+  val configurationName = "testAgp${version.replace(".", "")}x"
+}
+
+private val agpConfigurations = listOf(
+    AgpConfiguration("3.2", Libs.com_android_tools_build_gradle_32x),
+    AgpConfiguration("3.3", Libs.com_android_tools_build_gradle_33x),
+    AgpConfiguration("3.4", Libs.com_android_tools_build_gradle_34x),
+    AgpConfiguration("3.5", Libs.com_android_tools_build_gradle_35x)
+)
+
 configurations {
-  create("testAgp32x") {
-    description = "Local dependencies used for compiling & running " +
-        "tests source code in Gradle functional tests against AGP 3.2.X"
-    extendsFrom(configurations.getByName("implementation"))
-    resolutionStrategy.force(Libs.com_android_tools_build_gradle_32x)
-  }
-  create("testAgp33x") {
-    description = "Local dependencies used for compiling & running " +
-        "tests source code in Gradle functional tests against AGP 3.3.X"
-    extendsFrom(configurations.getByName("implementation"))
-    resolutionStrategy.force(Libs.com_android_tools_build_gradle_33x)
-  }
-  create("testAgp34x") {
-    description = "Local dependencies used for compiling & running " +
-        "tests source code in Gradle functional tests against AGP 3.4.X"
-    extendsFrom(configurations.getByName("implementation"))
-    resolutionStrategy.force(Libs.com_android_tools_build_gradle_34x)
-  }
-  create("testAgp35x") {
-    description = "Local dependencies used for compiling & running " +
-        "tests source code in Gradle functional tests against AGP 3.5.X"
-    extendsFrom(configurations.getByName("implementation"))
-    resolutionStrategy.force(Libs.com_android_tools_build_gradle_35x)
+  // Create a custom configuration for each version
+  agpConfigurations.forEach { agpConfig ->
+    create(agpConfig.configurationName) {
+      description = "Local dependencies used for compiling & running " +
+          "tests source code in Gradle functional tests against AGP ${agpConfig.version}"
+      extendsFrom(configurations.getByName("implementation"))
+      dependencies {
+        this@create(agpConfig.dependency)
+      }
+    }
   }
 }
 
@@ -179,15 +182,16 @@ tasks.withType<WriteClasspathResource> {
 // ------------------------------------------------------------------------------------------------
 
 dependencies {
+  compileOnly(Libs.com_android_tools_build_gradle)
   implementation(gradleApi())
-  implementation(Libs.com_android_tools_build_gradle)
   implementation(Libs.kotlin_gradle_plugin)
   implementation(Libs.kotlin_stdlib_jdk8)
   implementation(Libs.java_semver)
   implementation(Libs.stream)
   implementation(Libs.junit_platform_commons)
 
   testImplementation(gradleTestKit())
+  testImplementation(Libs.com_android_tools_build_gradle)
   testImplementation(Libs.commons_io)
   testImplementation(Libs.commons_lang)
   testImplementation(Libs.mockito_core)