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/gapi.go

@@ -25,20 +25,6 @@ import (
 	"github.com/purpleidea/mgmt/engine"
 	"github.com/purpleidea/mgmt/engine/local"
 	"github.com/purpleidea/mgmt/pgraph"
-
-	"github.com/urfave/cli/v2"
-)
-
-const (
-	// CommandRun is the identifier for the "run" command. It is distinct
-	// from the other commands, because it can run with any front-end.
-	CommandRun = "run"
-
-	// CommandDeploy is the identifier for the "deploy" command.
-	CommandDeploy = "deploy"
-
-	// CommandGet is the identifier for the "get" (download) command.
-	CommandGet = "get"
 )
 
 // RegisteredGAPIs is a global map of all possible GAPIs which can be used. You
@@ -55,15 +41,32 @@ func Register(name string, fn func() GAPI) {
 	RegisteredGAPIs[name] = fn
 }
 
-// CliInfo is the set of input values passed into the Cli method so that the
-// GAPI can decide if it wants to activate, and if it does, the initial handles
-// it needs to use to do so.
-type CliInfo struct {
-	// CliContext is the struct that is used to transfer in user input.
-	CliContext *cli.Context
+// Flags is some common data that comes from a higher-level command, and is used
+// by a subcommand. By type circularity, the subcommands can't easily access the
+// data in the parent command struct, so instead, the data that the parent wants
+// to pass down, it wraps up in a struct (for API convenience) and sends it out.
+type Flags struct {
+	Hostname *string
+
+	Noop bool
+	Sema int
+}
+
+// Info is the set of input values passed into the Cli method so that the GAPI
+// can decide if it wants to activate, and if it does, the initial handles it
+// needs to use to do so.
+type Info struct {
+	// Args are the CLI args that are populated after parsing the args list.
+	// They need to be converted to the struct you are expecting to read it.
+	Args interface{}
+
+	// Flags are the common data which is passed down into the sub command.
+	Flags *Flags
+
 	// Fs is the filesystem the Cli method should copy data into. It usually
 	// copies *from* the local filesystem using standard io functionality.
-	Fs    engine.Fs
+	Fs engine.Fs
+
 	Debug bool
 	Logf  func(format string, v ...interface{})
 }
@@ -99,18 +102,11 @@ type Next struct {
 // GAPI is a Graph API that represents incoming graphs and change streams. It is
 // the frontend interface that needs to be implemented to use the engine.
 type GAPI interface {
-	// CliFlags is passed a Command constant specifying which command it is
-	// requesting the flags for. If an invalid or unsupported command is
-	// passed in, simply return an empty list. Similarly, it is not required
-	// to ever return any flags, and the GAPI may always return an empty
-	// list.
-	CliFlags(string) []cli.Flag
-
 	// Cli is run on each GAPI to give it a chance to decide if it wants to
 	// activate, and if it does, then it will return a deploy struct. During
-	// this time, it uses the CliInfo struct as useful information to decide
+	// this time, it uses the Info struct as useful information to decide
 	// what to do.
-	Cli(*CliInfo) (*Deploy, error)
+	Cli(*Info) (*Deploy, error)
 
 	// Init initializes the GAPI and passes in some useful data.
 	Init(*Data) error
@@ -130,28 +126,3 @@ type GAPI interface {
 	// TODO: change Close to Cleanup
 	Close() error
 }
-
-// GetInfo is the set of input values passed into the Get method for it to run.
-type GetInfo struct {
-	// CliContext is the struct that is used to transfer in user input.
-	CliContext *cli.Context
-
-	Noop   bool
-	Sema   int
-	Update bool
-
-	Debug bool
-	Logf  func(format string, v ...interface{})
-}
-
-// GettableGAPI represents additional methods that need to be implemented in
-// this GAPI so that it can be used with the `get` Command. The methods in this
-// interface are called independently from the rest of the GAPI interface, and
-// you must not rely on shared state from those methods. Logically, this should
-// probably be named "Getable", however the correct modern word is "Gettable".
-type GettableGAPI interface {
-	GAPI // the base interface must be implemented
-
-	// Get runs the get/download method.
-	Get(*GetInfo) error
-}