configs: Allow looking up resources by resource addresses.

Throughout the main "terraform" package we identify resources using the
address types, and so this helper is useful to make concise transitions
between the address types and the configuration types.

As part of this, we use the address types to produce the keys used in our
resource maps. This has no visible change in behavior since the prior
implementation produced an equal result, but this change ensures that
ResourceByAddr cannot be broken by hypothetical future changes to the
key serialization.

configs/module.go

@@ -4,6 +4,8 @@ import (
 	"fmt"
 
 	"github.com/hashicorp/hcl2/hcl"
+
+	"github.com/hashicorp/terraform/addrs"
 )
 
 // Module is a container for a set of configuration constructs that are
@@ -99,6 +101,20 @@ func NewModule(primaryFiles, overrideFiles []*File) (*Module, hcl.Diagnostics) {
 	return mod, diags
 }
 
+// ResourceByAddr returns the configuration for the resource with the given
+// address, or nil if there is no such resource.
+func (m *Module) ResourceByAddr(addr addrs.Resource) *Resource {
+	key := addr.String()
+	switch addr.Mode {
+	case addrs.ManagedResourceMode:
+		return m.ManagedResources[key]
+	case addrs.DataResourceMode:
+		return m.DataResources[key]
+	default:
+		return nil
+	}
+}
+
 func (m *Module) appendFile(file *File) hcl.Diagnostics {
 	var diags hcl.Diagnostics
 

configs/resource.go

@@ -47,14 +47,7 @@ type ManagedResource struct {
 }
 
 func (r *Resource) moduleUniqueKey() string {
-	switch r.Mode {
-	case addrs.ManagedResourceMode:
-		return fmt.Sprintf("%s.%s", r.Name, r.Type)
-	case addrs.DataResourceMode:
-		return fmt.Sprintf("data.%s.%s", r.Name, r.Type)
-	default:
-		panic(fmt.Errorf("Resource has invalid resource mode %s", r.Mode))
-	}
+	return r.Addr().String()
 }
 
 // Addr returns a resource address for the receiver that is relative to the