Skip the execution of repackage in parent projects

If an execution of the repackage goal is present in a project with
'pom' packaging, it is now skipped. This allows to configure the
plugin in the parent.

Fixes gh-867

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

+<?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-pom</artifactId>
+	<packaging>pom</packaging>
+	<version>0.0.1.BUILD-SNAPSHOT</version>
+	<properties>
+		<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+	</properties>
+	<build>
+		<plugins>
+			<plugin>
+				<groupId>@project.groupId@</groupId>
+				<artifactId>@project.artifactId@</artifactId>
+				<version>@project.version@</version>
+				<executions>
+					<execution>
+						<goals>
+							<goal>repackage</goal>
+						</goals>
+					</execution>
+				</executions>
+			</plugin>
+		</plugins>
+	</build>
+	<dependencies>
+		<dependency>
+			<groupId>org.springframework</groupId>
+			<artifactId>spring-context</artifactId>
+			<version>3.2.3.RELEASE</version>
+		</dependency>
+		<dependency>
+			<groupId>javax.servlet</groupId>
+			<artifactId>javax.servlet-api</artifactId>
+			<version>3.0.1</version>
+			<scope>provided</scope>
+		</dependency>
+	</dependencies>
+</project>

spring-boot-tools/spring-boot-maven-plugin/src/main/java/org/springframework/boot/maven/RepackageMojo.java

@@ -110,6 +110,11 @@ public class RepackageMojo extends AbstractDependencyFilterMojo {
 
 	@Override
 	public void execute() throws MojoExecutionException, MojoFailureException {
+		if (project.getPackaging().equals("pom")) {
+			getLog().debug("repackage goal could not be applied to pom project.");
+			return;
+		}
+
 		File source = this.project.getArtifact().getFile();
 		File target = getTargetFile();
 		Repackager repackager = new Repackager(source) {