GH-157: Fix optional dependencies for pom

Fixes https://github.com/spring-projects/spring-integration-aws/issues/157

It turns out that `implementation` provides a `runtime` dependency in
Maven's POM, which is not what we would expect to behave as an `optional`

* Apply Gradle variants for those dependencies which should be as
`optional` in the generate POM
* Some Gradle build config refactoring

build.gradle

@@ -7,7 +7,7 @@ plugins {
 	id 'checkstyle'
 	id 'org.ajoberstar.grgit' version '4.0.0'
 	id "io.spring.dependency-management" version '1.0.8.RELEASE'
-	id 'com.jfrog.artifactory' version '4.10.0' apply false
+	id 'com.jfrog.artifactory' version '4.11.0'
 }
 
 description = 'Spring Integration AWS Support'
@@ -60,6 +60,9 @@ compileJava {
 java {
 	withJavadocJar()
 	withSourcesJar()
+	registerFeature('optional') {
+		usingSourceSet(sourceSets.main)
+	}
 }
 
 sourceSets {
@@ -100,18 +103,18 @@ dependencies {
 	api 'org.springframework.cloud:spring-cloud-aws-core'
 	api 'com.fasterxml.jackson.core:jackson-databind'
 
-	implementation 'org.springframework.cloud:spring-cloud-aws-messaging'
-	implementation 'org.springframework.integration:spring-integration-file'
-	implementation 'org.springframework.integration:spring-integration-http'
+	optionalApi 'org.springframework.cloud:spring-cloud-aws-messaging'
+	optionalApi 'org.springframework.integration:spring-integration-file'
+	optionalApi 'org.springframework.integration:spring-integration-http'
 
-	implementation "com.amazonaws:amazon-kinesis-client:$kinesisClientVersion"
-	implementation "com.amazonaws:amazon-kinesis-producer:$kinesisProducerVersion"
+	optionalApi "com.amazonaws:amazon-kinesis-client:$kinesisClientVersion"
+	optionalApi "com.amazonaws:amazon-kinesis-producer:$kinesisProducerVersion"
 
-	implementation 'com.amazonaws:aws-java-sdk-kinesis'
-	implementation 'com.amazonaws:aws-java-sdk-dynamodb'
-	implementation "com.amazonaws:dynamodb-lock-client:$dynamodbLockClientVersion"
+	optionalApi 'com.amazonaws:aws-java-sdk-kinesis'
+	optionalApi 'com.amazonaws:aws-java-sdk-dynamodb'
+	optionalApi "com.amazonaws:dynamodb-lock-client:$dynamodbLockClientVersion"
 
-	implementation "javax.servlet:javax.servlet-api:$servletApiVersion"
+	optionalApi "javax.servlet:javax.servlet-api:$servletApiVersion"
 
 	testImplementation ('org.springframework.integration:spring-integration-test') {
 		exclude group: 'junit'
@@ -231,7 +234,9 @@ task schemaZip(type: Zip) {
 	description = "Builds -${archiveClassifier} archive containing all " +
 			"XSDs for deployment at static.springframework.org/schema."
 
-	def Properties schemas = new Properties();
+	duplicatesStrategy = DuplicatesStrategy.EXCLUDE
+
+	Properties schemas = new Properties();
 	def shortName = idPrefix.replaceFirst("${idPrefix}-", '')
 
 	project.sourceSets.main.resources.find {

publish-maven.gradle

@@ -1,9 +1,9 @@
 apply plugin: 'maven-publish'
-apply plugin: 'com.jfrog.artifactory'
 
 publishing {
     publications {
         mavenJava(MavenPublication) {
+            suppressAllPomMetadataWarnings()
             from components.java
             artifact docsZip
             artifact schemaZip