Add buildSrc including build conventions plugins

Closes gh-1942

buildSrc/src/test/java/io/spring/gradle/convention/JavadocApiPluginITest.java

@@ -0,0 +1,38 @@
+package io.spring.gradle.convention;
+
+import io.spring.gradle.TestKit;
+import org.apache.commons.io.FileUtils;
+import org.gradle.testkit.runner.BuildResult;
+import org.gradle.testkit.runner.TaskOutcome;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+
+import java.io.File;
+import java.nio.file.Path;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class JavadocApiPluginITest {
+	private TestKit testKit;
+
+	@BeforeEach
+	void setup(@TempDir Path tempDir) {
+		this.testKit = new TestKit(tempDir.toFile());
+	}
+
+	@Test
+    public void multiModuleApi() throws Exception {
+        BuildResult result = this.testKit.withProjectResource("samples/javadocapi/multimodule/")
+				.withArguments("api")
+				.build();
+		assertThat(result.task(":api").getOutcome()).isEqualTo(TaskOutcome.SUCCESS);
+        File allClasses = new File(testKit.getRootDir(), "build/api/allclasses-noframe.html");
+		File index = new File(testKit.getRootDir(), "build/api/allclasses.html");
+		File listing = allClasses.exists() ? allClasses : index;
+		String listingText = FileUtils.readFileToString(listing);
+		assertThat(listingText).contains("sample/Api.html");
+        assertThat(listingText).contains("sample/Impl.html");
+		assertThat(listingText).doesNotContain("sample/Sample.html");
+	}
+}