Make artifact cache threadsafe.

Use singleton holder pattern for lazy initialisation of static cache, and make the cache concurrency safe as it can be read and updated simultaneously in multi module builds.

src/main/java/org/codehaus/mojo/license/api/DefaultThirdPartyHelper.java

@@ -44,6 +44,7 @@
 import java.util.SortedMap;
 import java.util.SortedSet;
 import java.util.TreeMap;
+import java.util.concurrent.ConcurrentSkipListMap;
 import org.eclipse.aether.repository.RemoteRepository;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -101,7 +102,12 @@ public class DefaultThirdPartyHelper
     /**
      * Cache of dependencies (as maven project) loaded.
      */
-    private static SortedMap<String, MavenProject> artifactCache;
+    private static final class ArtifactCacheHolder
+    {
+        private ArtifactCacheHolder() {}
+
+        private static final SortedMap<String, MavenProject> artifactCache = new ConcurrentSkipListMap<>();
+    }
 
     /**
      * Constructor of the helper.
@@ -136,11 +142,7 @@ public DefaultThirdPartyHelper( MavenProject project, String encoding, boolean v
      */
     public SortedMap<String, MavenProject> getArtifactCache()
     {
-        if ( artifactCache == null )
-        {
-            artifactCache = new TreeMap<>();
-        }
-        return artifactCache;
+        return ArtifactCacheHolder.artifactCache;
     }
 
     /**