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/src/test/java/org/springframework/modulith/core/util/ApplicationModulesExporterUnitTests.java

@@ -0,0 +1,42 @@
+/*
+ * Copyright 2023 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.modulith.core.util;
+
+import static org.assertj.core.api.Assertions.*;
+
+import org.junit.jupiter.api.Test;
+import org.springframework.modulith.core.ApplicationModules;
+
+import com.acme.myproject.Application;
+import com.fasterxml.jackson.databind.ObjectMapper;
+
+/**
+ * Unit tests for {@link ApplicationModulesExporter}.
+ *
+ * @author Oliver Drotbohm
+ */
+public class ApplicationModulesExporterUnitTests {
+
+	@Test // #119
+	void rendersApplicationModulesAsJson() {
+
+		var json = new ApplicationModulesExporter(ApplicationModules.of(Application.class)).toJson();
+
+		assertThatNoException().isThrownBy(() -> {
+			new ObjectMapper().readTree(json);
+		});
+	}
+}