Generate and attach dependency-tree

Update spring-boot-versions to generate a dependency-tree file and
attach it as an artifact. The file is generated by creating a temporary
POM and calling the invoker plugin.

The spring-boot-versions POM now depends on all spring-boot-starter-*
POMs to ensure that they have been installed before the dependency
tree is processes.

See gh-1047

spring-boot-versions/src/dependency-tree/pom.xml

@@ -0,0 +1,40 @@
+<?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>
+	<parent>
+		<groupId>org.springframework.boot</groupId>
+		<artifactId>spring-boot-versions</artifactId>
+		<version>@project.version@</version>
+		<relativePath>../../target/dependency-tree</relativePath>
+	</parent>
+	<artifactId>spring-boot-versions-dependency-tree</artifactId>
+	<packaging>pom</packaging>
+	<build>
+		<plugins>
+			<plugin>
+				<groupId>org.apache.maven.plugins</groupId>
+				<artifactId>maven-dependency-plugin</artifactId>
+				<executions>
+					<execution>
+						<id>write-dependencies-tree</id>
+						<phase>generate-resources</phase>
+						<goals>
+							<goal>tree</goal>
+						</goals>
+						<configuration>
+							<outputFile>@project.build.directory@/dependency-tree/dependency-tree.txt</outputFile>
+							<verbose>true</verbose>
+						</configuration>
+					</execution>
+				</executions>
+			</plugin>
+		</plugins>
+	</build>
+	<repositories>
+		<repository>
+			<id>spring-ext</id>
+			<url>http://repo.spring.io/ext-release-local/</url>
+		</repository>
+	</repositories>
+</project>