lang: Move StructTag const into lang/types

This constant value is strongly tied to the language, and little to do
with the engine. Move the definition into the lang/types package to
prevent circular imports between lang/types and engine/util.

Signed-off-by: Joe Groocock <me@frebib.net>

lang/types/type.go

@@ -25,6 +25,11 @@ import (
 	"github.com/purpleidea/mgmt/util/errwrap"
 )
 
+const (
+	// StructTag is the key we use in struct field names for key mapping.
+	StructTag = "lang"
+)
+
 // Basic types defined here as a convenience for use with Type.Cmp(X).
 var (
 	TypeBool    = NewType("bool")