Tolerate incomplete models

This commit improves the resolution of plugins to ignore a non fatal
resolution of the model.

See gh-1227

initializr-version-resolver/pom.xml

@@ -21,6 +21,10 @@
 	</scm>
 
 	<dependencies>
+		<dependency>
+			<groupId>org.springframework</groupId>
+			<artifactId>spring-jcl</artifactId>
+		</dependency>
 		<dependency>
 			<groupId>org.apache.maven</groupId>
 			<artifactId>maven-core</artifactId>

initializr-version-resolver/src/main/java/io/spring/initializr/versionresolver/DefaultMavenVersionResolver.java

@@ -22,6 +22,8 @@ import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 
+import org.apache.commons.logging.Log;
+import org.apache.commons.logging.LogFactory;
 import org.apache.maven.model.Model;
 import org.apache.maven.model.building.DefaultModelBuilder;
 import org.apache.maven.model.building.DefaultModelBuilderFactory;
@@ -68,6 +70,8 @@ import org.eclipse.aether.util.repository.SimpleArtifactDescriptorPolicy;
 @SuppressWarnings("removal")
 class DefaultMavenVersionResolver implements MavenVersionResolver, DependencyManagementVersionResolver {
 
+	private static final Log logger = LogFactory.getLog(DefaultMavenVersionResolver.class);
+
 	private static final RemoteRepository mavenCentral = new RemoteRepository.Builder("central", "default",
 			"https://repo1.maven.org/maven2")
 		.build();
@@ -163,6 +167,12 @@ class DefaultMavenVersionResolver implements MavenVersionResolver, DependencyMan
 			return modelBuilder.build(modelBuildingRequest).getEffectiveModel();
 		}
 		catch (ModelBuildingException ex) {
+			Model model = ex.getModel();
+			if (model != null) {
+				logger.warn("Model for '" + groupId + ":" + artifactId + ":" + version + "' is incomplete: "
+						+ ex.getProblems());
+				return model;
+			}
 			throw new IllegalStateException(
 					"Model for '" + groupId + ":" + artifactId + ":" + version + "' could not be built", ex);
 		}

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

@@ -90,6 +90,13 @@ class DefaultMavenVersionResolverTests {
 		assertThat(versions).containsEntry("org.springframework.boot:spring-boot-maven-plugin", "2.2.0.M3");
 	}
 
+	@Test
+	void resolvePluginsUsingMilestoneThatHasResolutionProblem() {
+		Map<String, String> versions = this.resolver.resolvePlugins("org.springframework.boot",
+				"spring-boot-dependencies", "3.0.0-M1");
+		assertThat(versions).containsEntry("org.springframework.boot:spring-boot-maven-plugin", "3.0.0-M1");
+	}
+
 	@Test
 	void resolvePluginsUsingSnapshots() {
 		Map<String, String> versions = this.resolver.resolvePlugins("org.springframework.boot",