internal/dag: adapt Validate to not check for Root

The implementation of the DAG as extracted from Terraform relied on a
Root vertex being injected into the graph as the last node to visit.

This is used as a sanity check for Terraform, but doesn't apply to our
use-case for now, as we are always executing everything and have no need
for this root node.

Instead, we change how Validate operates so it does not error in case
there is no valid root node for the graph, but enables us calling it to
check for self-referencing edges, and circular dependencies.

hcl2template/parser.go

@@ -420,6 +420,10 @@ func (cfg *PackerConfig) buildPrereqsDAG() (*dag.AcyclicGraph, error) {
 		}
 	}
 
+	if err := retGraph.Validate(); err != nil {
+		return nil, err
+	}
+
 	return &retGraph, nil
 }
 

internal/dag/dag.go

@@ -127,13 +127,8 @@ func (g *AcyclicGraph) TransitiveReduction() {
 	}
 }
 
-// Validate validates the DAG. A DAG is valid if it has a single root
-// with no cycles.
+// Validate validates the DAG. A DAG is valid if it has no cycles or self-referencing vertex.
 func (g *AcyclicGraph) Validate() error {
-	if _, err := g.Root(); err != nil {
-		return err
-	}
-
 	// Look for cycles of more than 1 component
 	var err error
 	cycles := g.Cycles()