hcl2template: split local evaluation functions

Since we are in the process of integrating a new way to orchestrate
dependency management and evaluation for datsources and local variables,
we need to split the current function that manages the evaluation of
said local variables, so that recursion and the actual evaluation of a
local variable are two separate functions.

This will allow for evaluating a single variable once the dag is ready
to be introduced.

hcl2template/types.packer_config.go

@@ -274,7 +274,7 @@ func (c *PackerConfig) evaluateLocalVariables(locals []*LocalBlock) hcl.Diagnost
 	}
 
 	for _, local := range c.LocalBlocks {
-		diags = diags.Extend(c.evaluateLocalVariable(local, 0))
+		diags = diags.Extend(c.recursivelyEvaluateLocalVariable(local, 0))
 	}
 
 	return diags
@@ -306,7 +306,7 @@ func (c *PackerConfig) checkForDuplicateLocalDefinition() hcl.Diagnostics {
 	return diags
 }
 
-func (c *PackerConfig) evaluateLocalVariable(local *LocalBlock, depth int) hcl.Diagnostics {
+func (c *PackerConfig) recursivelyEvaluateLocalVariable(local *LocalBlock, depth int) hcl.Diagnostics {
 	// If the variable already was evaluated, we can return immediately
 	if local.evaluated {
 		return nil
@@ -325,11 +325,17 @@ func (c *PackerConfig) evaluateLocalVariable(local *LocalBlock, depth int) hcl.D
 	var diags hcl.Diagnostics
 
 	for _, dep := range local.dependencies {
-		localDiags := c.evaluateLocalVariable(dep, depth+1)
+		localDiags := c.recursivelyEvaluateLocalVariable(dep, depth+1)
 		diags = diags.Extend(localDiags)
 	}
 
-	value, moreDiags := local.Expr.Value(c.EvalContext(LocalContext, nil))
+	return diags.Extend(c.evaluateLocalVariable(local))
+}
+
+func (cfg *PackerConfig) evaluateLocalVariable(local *LocalBlock) hcl.Diagnostics {
+	var diags hcl.Diagnostics
+
+	value, moreDiags := local.Expr.Value(cfg.EvalContext(LocalContext, nil))
 
 	local.evaluated = true
 
@@ -337,7 +343,7 @@ func (c *PackerConfig) evaluateLocalVariable(local *LocalBlock, depth int) hcl.D
 	if moreDiags.HasErrors() {
 		return diags
 	}
-	c.LocalVariables[local.Name] = &Variable{
+	cfg.LocalVariables[local.Name] = &Variable{
 		Name:      local.Name,
 		Sensitive: local.Sensitive,
 		Values: []VariableAssignment{{