GH-119 - Improve rendering of application modules structure as JSON.

Introduced ApplicationModulesExporter to render an ApplicationModules instances as JSON directly. To avoid a dependency to a JSON library and as we only have to be able to render rather simple arrangements, we just build up the JSON string ourselves.

ApplicationModulesEndpoint now caches the structure calculated once to avoid repeated work.

spring-modulith-integration-test/pom.xml

@@ -52,12 +52,20 @@
 			<artifactId>spring-tx</artifactId>
 		</dependency>
 
+		<!-- Test -->
+
 		<dependency>
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-test</artifactId>
 			<scope>test</scope>
 		</dependency>
-		
+
+		<dependency>
+			<groupId>com.fasterxml.jackson.core</groupId>
+			<artifactId>jackson-databind</artifactId>
+			<scope>test</scope>
+		</dependency>
+
 		<dependency>
 			<groupId>org.jgrapht</groupId>
 			<artifactId>jgrapht-core</artifactId>