Relax plugin id url heuristic, since java uses single slash instead of double

core/src/main/java/org/elasticsearch/plugins/InstallPluginCommand.java

@@ -237,7 +237,7 @@ class InstallPluginCommand extends SettingCommand {
         }
 
         // fall back to plain old URL
-        if (pluginId.contains("://") == false) {
+        if (pluginId.contains(":/") == false) {
             // definitely not a valid url, so assume it is a plugin name
             throw new UserError(ExitCodes.USAGE, "Unknown plugin " + pluginId);
         }