hcl2template: intro and add UseDAG init opt

Following up on the DAG work, this commit adds a new option for
initialisation that enables DAG if required.

By default we are still going to use the current phased approach, so we
don't risk breaking existing configurations.

However, this option can then be exposed as a command-line flag for
people wanting to experiment with this evaluation technique and the
benefits it brings.

hcl2template/parser.go

@@ -529,8 +529,13 @@ func (cfg *PackerConfig) evaluateBuildPrereqs(skipDatasources bool) hcl.Diagnost
 
 func (cfg *PackerConfig) Initialize(opts packer.InitializeOptions) hcl.Diagnostics {
 	diags := cfg.InputVariables.ValidateValues()
-	diags = append(diags, cfg.evaluateDatasources(opts.SkipDatasourcesExecution)...)
-	diags = append(diags, cfg.evaluateLocalVariables(cfg.LocalBlocks)...)
+
+	if opts.UseDAG {
+		diags = diags.Extend(cfg.evaluateBuildPrereqs(opts.SkipDatasourcesExecution))
+	} else {
+		diags = diags.Extend(cfg.evaluateDatasources(opts.SkipDatasourcesExecution))
+		diags = diags.Extend(cfg.evaluateLocalVariables(cfg.LocalBlocks))
+	}
 
 	filterVarsFromLogs(cfg.InputVariables)
 	filterVarsFromLogs(cfg.LocalVariables)

packer/run_interfaces.go

@@ -38,6 +38,14 @@ type InitializeOptions struct {
 	// When set, the execution of datasources will be skipped and the datasource will provide
 	// an output spec that will be used for validation only.
 	SkipDatasourcesExecution bool
+	// UseDAG changes the way data sources and locals are evaluated.
+	//
+	// In this mode, instead of using two separate phases to evaluate datasources first, then
+	// local variables, here we instead use a DAG so both are evaluated at once, based on the
+	// dependencies between them.
+	//
+	// This is optional and defaults to false for now, but this may become a default later.
+	UseDAG bool
 }
 
 type PluginBinaryDetector interface {