gapi: Add a new Names API

This gets the list more directly. We could also just lookup a key in the
big map to get the same effect, but this is more logical for now. This
is useful for removing the importing of three packages.

cli/deploy.go

@@ -32,6 +32,7 @@ import (
 	emptyGAPI "github.com/purpleidea/mgmt/gapi/empty"
 	langGAPI "github.com/purpleidea/mgmt/lang/gapi"
 	"github.com/purpleidea/mgmt/lib"
+	"github.com/purpleidea/mgmt/util"
 	"github.com/purpleidea/mgmt/util/errwrap"
 	yamlGAPI "github.com/purpleidea/mgmt/yamlgraph"
 
@@ -80,9 +81,10 @@ func (obj *DeployArgs) Run(ctx context.Context, data *cliUtil.Data) (bool, error
 	}
 
 	// XXX: workaround https://github.com/alexflint/go-arg/issues/239
+	gapiNames := gapi.Names() // list of registered names
 	if l := len(obj.Seeds); name == "" && l > 1 {
 		elem := obj.Seeds[l-2] // second to last element
-		if elem == emptyGAPI.Name || elem == langGAPI.Name || elem == yamlGAPI.Name {
+		if util.StrInList(elem, gapiNames) {
 			return false, cliUtil.CliParseError(cliUtil.MissingEquals) // consistent errors
 		}
 	}

cli/run.go

@@ -129,10 +129,11 @@ func (obj *RunArgs) Run(ctx context.Context, data *cliUtil.Data) (bool, error) {
 		obj.AdvertiseClientURLs,
 		obj.AdvertiseServerURLs,
 	}
+	gapiNames := gapi.Names() // list of registered names
 	for _, list := range lists {
 		if l := len(list); name == "" && l > 1 {
 			elem := list[l-2] // second to last element
-			if elem == emptyGAPI.Name || elem == langGAPI.Name || elem == yamlGAPI.Name {
+			if util.StrInList(elem, gapiNames) {
 				return false, cliUtil.CliParseError(cliUtil.MissingEquals) // consistent errors
 			}
 		}

gapi/gapi.go

@@ -41,6 +41,15 @@ func Register(name string, fn func() GAPI) {
 	RegisteredGAPIs[name] = fn
 }
 
+// Names returns a list of the names of all registered GAPIs.
+func Names() []string {
+	names := []string{}
+	for name := range RegisteredGAPIs {
+		names = append(names, name)
+	}
+	return names
+}
+
 // 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