lang: Move the Edge struct into the interfaces package

This makes it consumable from more than one package and avoids future
cycles.

lang/interfaces/func.go

@@ -18,6 +18,8 @@
 package interfaces
 
 import (
+	"strings"
+
 	"github.com/purpleidea/mgmt/engine"
 	"github.com/purpleidea/mgmt/lang/types"
 )
@@ -187,3 +189,15 @@ type DataFunc interface {
 	// context.
 	SetData(*FuncData)
 }
+
+// FuncEdge links an output vertex (value) to an input vertex with a named
+// argument.
+type FuncEdge struct {
+	Args []string // list of named args that this edge sends to
+}
+
+// String displays the list of arguments this edge satisfies. It is a required
+// property to be a valid pgraph.Edge.
+func (obj *FuncEdge) String() string {
+	return strings.Join(obj.Args, ", ")
+}