Provide group in ResolvedArtifactLibrary to enable discrimination

The fix for gh-1475 introduced the use of an artifact's group to
discriminate between two libraries with the same name (artifact id)
and version. However, in the case of Gradle, a group name was not
provided for libraries that have been resolved from a repository.

This commit updates ResolvedArtifactLibrary to use the group obtained
from the underlying ResolvedArtifact as its discriminator.

Fixes gh-1543

spring-boot-tools/spring-boot-gradle-plugin/src/main/groovy/org/springframework/boot/gradle/repackage/ProjectLibraries.java

@@ -209,7 +209,8 @@ class ProjectLibraries implements Libraries {
 		private final ResolvedArtifact artifact;
 
 		public ResolvedArtifactLibrary(ResolvedArtifact artifact, LibraryScope scope) {
-			super(null, artifact.getFile(), scope);
+			super(artifact.getModuleVersion().getId().getGroup(), artifact.getFile(),
+					scope);
 			this.artifact = artifact;
 		}