Allow repackage maven goal to take a source classifier

Previously, the "classifier" attribute was only used to determine the
target classifier of the repackaged archive, always using the main
artifact as the source.

This commit changes the semantic of the attribute so that an existing
archive matching the "classifier" attribute can be used as source,
replacing the archive the same way the goal replaces the main archive
if no classifier is found.

If no artifact with the specified classifier exists, the repackaged
archive is still processed based on the main archive and attached to
the lifecycle using the value of the classifier attribute.

See gh-11061

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/it/jar-repackage-classifier/pom.xml

@@ -0,0 +1,66 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+	<modelVersion>4.0.0</modelVersion>
+	<groupId>org.springframework.boot.maven.it</groupId>
+	<artifactId>jar-repackage-classifier</artifactId>
+	<version>0.0.1.BUILD-SNAPSHOT</version>
+	<properties>
+		<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+		<maven.compiler.source>@java.version@</maven.compiler.source>
+		<maven.compiler.target>@java.version@</maven.compiler.target>
+	</properties>
+	<build>
+		<plugins>
+			<plugin>
+				<groupId>org.apache.maven.plugins</groupId>
+				<artifactId>maven-jar-plugin</artifactId>
+				<version>@maven-jar-plugin.version@</version>
+				<executions>
+					<execution>
+						<goals>
+							<goal>jar</goal>
+						</goals>
+						<phase>package</phase>
+						<configuration>
+							<classifier>test</classifier>
+							<archive>
+								<manifestEntries>
+									<Not-Used>Foo</Not-Used>
+								</manifestEntries>
+							</archive>
+						</configuration>
+					</execution>
+				</executions>
+			</plugin>
+			<plugin>
+				<groupId>@project.groupId@</groupId>
+				<artifactId>@project.artifactId@</artifactId>
+				<version>@project.version@</version>
+				<executions>
+					<execution>
+						<goals>
+							<goal>repackage</goal>
+						</goals>
+						<configuration>
+							<classifier>test</classifier>
+						</configuration>
+					</execution>
+				</executions>
+			</plugin>
+		</plugins>
+	</build>
+	<dependencies>
+		<dependency>
+			<groupId>org.springframework</groupId>
+			<artifactId>spring-context</artifactId>
+			<version>@spring.version@</version>
+		</dependency>
+		<dependency>
+			<groupId>javax.servlet</groupId>
+			<artifactId>javax.servlet-api</artifactId>
+			<version>@servlet-api.version@</version>
+			<scope>provided</scope>
+		</dependency>
+	</dependencies>
+</project>

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/it/jar-repackage-classifier/src/main/java/org/test/SampleApplication.java

@@ -0,0 +1,24 @@
+/*
+ * Copyright 2012-2014 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
+ *
+ *      http://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.test;
+
+public class SampleApplication {
+
+	public static void main(String[] args) {
+	}
+
+}

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/it/jar-repackage-classifier/verify.groovy

@@ -0,0 +1,26 @@
+/*
+ * Copyright 2012-2014 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
+ *
+ *      http://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.
+ */
+
+import java.io.*;
+import org.springframework.boot.maven.*;
+
+File f = new File(basedir, "target/jar-repackage-classifier-0.0.1.BUILD-SNAPSHOT-test.jar");
+new Verify.JarArchiveVerification(f, Verify.SAMPLE_APP) {
+	@Override
+	protected void verifyZipEntries(Verify.ArchiveVerifier verifier) throws Exception {
+		super.verifyZipEntries(verifier)
+	}
+}.verify();