hcl2template: fix func to get vars from a config

The previous implementation of the GetVarsByType function worked only on
top-level attributes, ignoring the nested blocks in the structure.

This implies that if a datasource depends on another through an
expression within a nested block, we may not execute it first, and then
executing this datasource before its dependent is possible, resulting in
an error in the end.

This commit is an attempt at making this more reliable for HCL configs,
but only works on configs lifted from HCL files for now. We need to make
this more reliable for later iterations.

hcl2template/utils.go

@@ -11,6 +11,7 @@ import (
 
 	"github.com/gobwas/glob"
 	"github.com/hashicorp/hcl/v2"
+	"github.com/hashicorp/hcl/v2/hclsyntax"
 	"github.com/hashicorp/packer/hcl2template/repl"
 	hcl2shim "github.com/hashicorp/packer/hcl2template/shim"
 	"github.com/zclconf/go-cty/cty"
@@ -187,22 +188,47 @@ func ConvertPluginConfigValueToHCLValue(v interface{}) (cty.Value, error) {
 	return buildValue, nil
 }
 
+// GetVarsByType walks through a hcl body, and gathers all the Traversals that
+// have a root type matching one of the specified top-level labels.
+//
+// This will only work on finite, expanded, HCL bodies.
 func GetVarsByType(block *hcl.Block, topLevelLabels ...string) []hcl.Traversal {
-	attributes, _ := block.Body.JustAttributes()
+	var travs []hcl.Traversal
 
-	var vars []hcl.Traversal
+	switch body := block.Body.(type) {
+	case *hclsyntax.Body:
+		travs = getVarsByTypeForHCLSyntaxBody(body)
+	default:
+		attrs, _ := body.JustAttributes()
+		for _, attr := range attrs {
+			travs = append(travs, attr.Expr.Variables()...)
+		}
+	}
 
-	for _, attr := range attributes {
-		for _, variable := range attr.Expr.Variables() {
-			rootLabel := variable.RootName()
-			for _, label := range topLevelLabels {
-				if label == rootLabel {
-					vars = append(vars, variable)
-					break
-				}
+	var rets []hcl.Traversal
+	for _, t := range travs {
+		varRootname := t.RootName()
+		for _, lbl := range topLevelLabels {
+			if varRootname == lbl {
+				rets = append(rets, t)
+				break
 			}
 		}
 	}
 
-	return vars
+	return rets
+}
+
+func getVarsByTypeForHCLSyntaxBody(body *hclsyntax.Body) []hcl.Traversal {
+	var rets []hcl.Traversal
+
+	for _, attr := range body.Attributes {
+		rets = append(rets, attr.Expr.Variables()...)
+	}
+
+	for _, block := range body.Blocks {
+		rets = append(rets, getVarsByTypeForHCLSyntaxBody(block.Body)...)
+	}
+
+	return rets
 }