Provide support for resolving versions from Maven boms

This commit introduces a new module, initializer-version-resolver,
that provides support for resolving versions from the dependency
management of a Maven bom. Providing with the bom's coordinates
(groupId, artifactId, and version) a map of groupId:artifactId to
version, derived from the bom's <dependencyManagement>, is returned.

Closes gh-934

initializr-version-resolver/src/test/java/io/spring/initializr/versionresolver/MavenResolverDependencyManagementVersionResolverTests.java

@@ -0,0 +1,78 @@
+/*
+ * Copyright 2012-2019 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
+ *
+ *      https://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 io.spring.initializr.versionresolver;
+
+import java.nio.file.Path;
+import java.util.Map;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
+
+/**
+ * Tests for {@link MavenResolverDependencyManagementVersionResolver}.
+ *
+ * @author Andy Wilkinson
+ */
+class MavenResolverDependencyManagementVersionResolverTests {
+
+	private DependencyManagementVersionResolver resolver;
+
+	@BeforeEach
+	void createResolver(@TempDir Path temp) {
+		this.resolver = new MavenResolverDependencyManagementVersionResolver(temp);
+	}
+
+	@Test
+	void springBootDependencies() {
+		Map<String, String> versions = this.resolver.resolve("org.springframework.boot", "spring-boot-dependencies",
+				"2.1.5.RELEASE");
+		assertThat(versions).containsEntry("org.flywaydb:flyway-core", "5.2.4");
+	}
+
+	@Test
+	void springCloudDependencies() {
+		Map<String, String> versions = this.resolver.resolve("org.springframework.cloud", "spring-cloud-dependencies",
+				"Greenwich.SR1");
+		assertThat(versions).containsEntry("com.netflix.ribbon:ribbon", "2.3.0");
+	}
+
+	@Test
+	void milestoneBomCanBeResolved() {
+		Map<String, String> versions = this.resolver.resolve("org.springframework.boot", "spring-boot-dependencies",
+				"2.2.0.M3");
+		assertThat(versions).containsEntry("org.flywaydb:flyway-core", "5.2.4");
+	}
+
+	@Test
+	void snapshotBomCanBeResolved() {
+		Map<String, String> versions = this.resolver.resolve("org.springframework.boot", "spring-boot-dependencies",
+				"2.2.0.BUILD-SNAPSHOT");
+		assertThat(versions).isNotEmpty();
+	}
+
+	@Test
+	void nonExistentDependency() {
+		assertThatIllegalStateException()
+				.isThrownBy(() -> this.resolver.resolve("org.springframework.boot", "spring-boot-bom", "1.0"))
+				.withMessage("Bom 'org.springframework.boot:spring-boot-bom:1.0' could not be resolved");
+	}
+
+}