dependency-manager: make it a singleton (#901)

Avoids double initialization in the agent, might save some bytes in
memory/on disk.

dependency-manager/src/main/java/com/walmartlabs/concord/dependencymanager/DependencyManager.java

@@ -50,6 +50,7 @@
 import org.slf4j.LoggerFactory;
 
 import javax.inject.Inject;
+import javax.inject.Singleton;
 import javax.xml.bind.DatatypeConverter;
 import java.io.IOException;
 import java.io.InputStream;
@@ -64,6 +65,7 @@
 import java.util.concurrent.Callable;
 import java.util.stream.Collectors;
 
+@Singleton
 public class DependencyManager {
 
     private static final Logger log = LoggerFactory.getLogger(DependencyManager.class);