cli-plugins: alias an existing allowed command (only builder for now)

With this patch it is possible to alias an existing allowed command.
At the moment only builder allows an alias.

This also properly puts the build command under builder, instead of image
where it was for historical reasons.

Signed-off-by: Tibor Vass <tibor@docker.com>

cli-plugins/manager/manager.go

@@ -164,6 +164,7 @@ func PluginRunCommand(dockerCli command.Cli, name string, rootcmd *cobra.Command
 			return nil, err
 		}
 		if plugin.Err != nil {
+			// TODO: why are we not returning plugin.Err?
 			return nil, errPluginNotFound(name)
 		}
 		cmd := exec.Command(plugin.Path, args...)