install/uninstall: allow aliasing buildx to docker builder

Currently the user can type `docker buildx` to use this tool.
This patch allows the user to install buildx as a `docker builder` alias.

As an additional benefit, this allows the regular `docker build` to hook
into `buildx build`.

Note that the install and uninstall commands are currently hidden.

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

vendor/github.com/docker/cli/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...)