feat(android): module builds should fail with aar in lib folder

- Breaking change as of Titanium 9.0.0
- Gradle build system won't merge AAR library into build AAR. So, while it compiles, the built AAR is missing classes the app needs to function.

android/templates/module/generated/build.gradle

@@ -120,20 +120,34 @@ android {
 			}
 		}
 	}
-
 	compileOptions {
 		sourceCompatibility JavaVersion.VERSION_1_8
 		targetCompatibility JavaVersion.VERSION_1_8
 	}
+	kotlinOptions {
+		jvmTarget = '1.8'
+	}
 	// Exclude the C++ runtime and Titanium kroll libraries for all architectures when packaging the AAR file.
 	// Avoids app build failure due to library file name collision. (Apps will use "titanium.aar" bundled *.so files.)
 	packagingOptions {
 		exclude '/lib/**/libc++_shared.so'
 		exclude '/lib/**/libkroll-v8.so'
 	}
-	// Sets the JVM target in order to allow usage of Java 8 features in Kotlin native modules.
-	kotlinOptions {
-		jvmTarget = '1.8'
+}
+
+// Trigger a build failure if an AAR library was found under the "lib" directory.
+// While it will compile with the library, Google's build system won't merge the AAR into this module's AAR.
+preBuild.doFirst {
+	def libDirectory = new File("${projectDir}/../../lib")
+	if (libDirectory.exists()) {
+		libDirectory.eachFile {
+			if (it.name.toLowerCase().endsWith('.aar')) {
+				def errorMessage =
+					'AAR files are no longer supported under the "lib" directory. ' +
+					'You must reference AAR libraries via a "build.gradle" file\'s dependencies section instead.'
+				throw new GradleException(errorMessage)
+			}
+		}
 	}
 }
 
@@ -169,12 +183,13 @@ dependencies {
 	compileOnly files(krollAptJarPath)
 	kapt files(krollAptJarPath)
 
-	// Reference this module's local AAR/JAR file dependencies.
-	implementation fileTree(dir: "${projectDir}/../../lib", include: ['*.aar', '*.jar'])
+	// Reference this module's local JAR file dependencies.
+	implementation fileTree(dir: "${projectDir}/../../lib", include: ['*.jar'])
 
 	// Reference the main Titanium library.
 	implementation 'org.appcelerator:titanium:<%- tiSdkVersion %>'
 
+	// Reference the core kotlin library.
 	implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk8:$kotlin_version"
 }