hcl2template: use refString for local/data deps

When registering dependencies for datasources and locals, we now use
refString.

This allows for the functions that detect dependencies to not only be
able to register the same types as dependencies, but instead generalises
it to any type that refString supports, so data, local and var.

This can then be leveraged for orchestrating evaluation of those
components in a non-phased way (i.e. with a DAG for dependency
management).

hcl2template/types.datasource.go

@@ -17,6 +17,7 @@ import (
 type DatasourceBlock struct {
 	Type         string
 	DSName       string
+	Dependencies []refString
 
 	value cty.Value
 	block *hcl.Block

hcl2template/types.packer_config.go

@@ -245,25 +245,27 @@ func (c *PackerConfig) evaluateLocalVariables(locals []*LocalBlock) hcl.Diagnost
 		// attributes, as HCL2 expressions are not allowed in a block's labels.
 		vars := FilterTraversalsByType(local.Expr.Variables(), "local")
 
-		var localDeps []*LocalBlock
+		var localDeps []refString
 		for _, v := range vars {
 			// Some local variables may be locally aliased as
 			// `local`, which
 			if len(v) < 2 {
 				continue
 			}
-			varName := v[1].(hcl.TraverseAttr).Name
-			block, err := c.localByName(varName)
+
+			depRef, err := NewRefStringFromDep(v)
 			if err != nil {
 				diags = diags.Append(&hcl.Diagnostic{
 					Severity: hcl.DiagError,
-					Summary:  "Missing variable dependency",
-					Detail: fmt.Sprintf("The expression for variable %q depends on local.%s, which is not defined.",
-						local.Name, varName),
+					Summary:  "failed to extract dependency name from traversal ref",
+					Detail: fmt.Sprintf("while preparing for evaluation of local variable %q, "+
+						"a dependency was unable to be converted to a refString. "+
+						"This is likely a Packer bug, please consider reporting it.", local.Name),
 				})
 				continue
 			}
-			localDeps = append(localDeps, block)
+
+			localDeps = append(localDeps, depRef)
 		}
 		local.dependencies = localDeps
 	}
@@ -325,7 +327,17 @@ func (c *PackerConfig) recursivelyEvaluateLocalVariable(local *LocalBlock, depth
 	var diags hcl.Diagnostics
 
 	for _, dep := range local.dependencies {
-		localDiags := c.recursivelyEvaluateLocalVariable(dep, depth+1)
+		locBlock, err := c.getComponentByRef(dep)
+		if err != nil {
+			return diags.Append(&hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  "failed to get local variable",
+				Detail: fmt.Sprintf("While evaluating %q, its dependency %q was not found, is it defined?",
+					local.Name, dep.String()),
+			})
+		}
+
+		localDiags := c.recursivelyEvaluateLocalVariable(locBlock.(*LocalBlock), depth+1)
 		diags = diags.Extend(localDiags)
 	}
 

hcl2template/types.variables.go

@@ -35,7 +35,7 @@ type LocalBlock struct {
 	//
 	// Only `local`/`locals` will be referenced here as we execute all the
 	// same component types at once.
-	dependencies []*LocalBlock
+	dependencies []refString
 	// evaluated toggles to true if it has been evaluated.
 	//
 	// We use this to determine if we're ready to get the value of the