cli, gapi: empty, lang, yaml: Refactor the args structs

Put these datastructures into an external package so they can be re-used
for parsing elsewhere.

Since we remove these dependencies, we need to manually import the
GAPI's so that they register. Despite efforts to embed them deeper into
the import tree without cycles, this failed. Logically what this told me
is that it actually makes sense to allow a different binary with only
one of the multiple GAPI's contained within.

main.go

@@ -25,6 +25,9 @@ import (
 
 	"github.com/purpleidea/mgmt/cli"
 	cliUtil "github.com/purpleidea/mgmt/cli/util"
+	_ "github.com/purpleidea/mgmt/gapi/empty" // import so the gapi registers
+	_ "github.com/purpleidea/mgmt/lang/gapi"  // import so the gapi registers
+	_ "github.com/purpleidea/mgmt/yamlgraph"  // import so the gapi registers
 	"go.etcd.io/etcd/server/v3/etcdmain"
 )