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!

examples/lib/libmgmt-subgraph0.go

@@ -16,8 +16,6 @@ import (
 	mgmt "github.com/purpleidea/mgmt/lib"
 	"github.com/purpleidea/mgmt/pgraph"
 	"github.com/purpleidea/mgmt/util/errwrap"
-
-	"github.com/urfave/cli/v2"
 )
 
 // XXX: this has not been updated to latest GAPI/Deploy API. Patches welcome!
@@ -51,31 +49,20 @@ func NewMyGAPI(data *gapi.Data, name string, interval uint) (*MyGAPI, error) {
 	return obj, obj.Init(data)
 }
 
-// CliFlags returns a list of flags used by the passed in subcommand.
-func (obj *MyGAPI) CliFlags(string) []cli.Flag {
-	return []cli.Flag{
-		&cli.StringFlag{
-			Name:  obj.Name,
-			Value: "",
-			Usage: "run",
-		},
-	}
-}
-
-// Cli takes a cli.Context and some other info, and returns our GAPI. If there
-// are any validation problems, you should return an error.
-func (obj *MyGAPI) Cli(cliInfo *gapi.CliInfo) (*gapi.Deploy, error) {
-	c := cliInfo.CliContext
-	//fs := cliInfo.Fs // copy files from local filesystem *into* this fs...
-	//debug := cliInfo.Debug
+// 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 *MyGAPI) Cli(info *gapi.CliInfo) (*gapi.Deploy, error) {
+	//fs := info.Fs // copy files from local filesystem *into* this fs...
+	//debug := info.Debug
 	//logf := func(format string, v ...interface{}) {
-	//	cliInfo.Logf(Name+": "+format, v...)
+	//	info.Logf(Name+": "+format, v...)
 	//}
 
 	return &gapi.Deploy{
 		Name: obj.Name,
-		Noop: c.Bool("noop"),
-		Sema: c.Int("sema"),
+		Noop: info.Flags.Noop,
+		Sema: info.Flags.Sema,
 		GAPI: &MyGAPI{},
 	}, nil
 }