cli, lib, lang: Port to new cli library

The new version of the urfave/cli library is moving to generics, and
it's completely unclear to me why this is an improvement. Their new API
is very complicated to understand, which for me, defeats the purpose of
golang.

In parallel, I needed to do some upcoming cli API refactoring, so this
was a good time to look into new libraries. After a review of the
landscape, I found the alexflint/go-arg library which has a delightfully
elegant API. It does have a few rough edges, but it's otherwise very
usable, and I think it would be straightforward to add features and fix
issues.

Thanks Alex!

gapi/empty/empty.go

@@ -23,8 +23,6 @@ import (
 
 	"github.com/purpleidea/mgmt/gapi"
 	"github.com/purpleidea/mgmt/pgraph"
-
-	"github.com/urfave/cli/v2"
 )
 
 const (
@@ -36,6 +34,9 @@ func init() {
 	gapi.Register(Name, func() gapi.GAPI { return &GAPI{} }) // register
 }
 
+// Args is the CLI parsing structure and type of the parsed result.
+type Args struct{}
+
 // GAPI implements the main lang GAPI interface.
 type GAPI struct {
 	data        *gapi.Data
@@ -44,16 +45,10 @@ type GAPI struct {
 	wg          *sync.WaitGroup // sync group for tunnel go routines
 }
 
-// CliFlags returns a list of flags used by the specified subcommand.
-func (obj *GAPI) CliFlags(command string) []cli.Flag {
-	return []cli.Flag{}
-}
-
-// Cli takes a cli.Context, and returns our GAPI if activated. All arguments
-// should take the prefix of the registered name. On activation, if there are
-// any validation problems, you should return an error. If this was not
-// activated, then you should return a nil GAPI and a nil error.
-func (obj *GAPI) Cli(*gapi.CliInfo) (*gapi.Deploy, error) {
+// Cli takes an *Info struct, and returns our deploy if activated, and if there
+// are any validation problems, you should return an error. If there is no
+// deploy, then you should return a nil deploy and a nil error.
+func (obj *GAPI) Cli(*gapi.Info) (*gapi.Deploy, error) {
 	return &gapi.Deploy{
 		Name: Name,
 		//Noop: false,