module: add path to the module object

This adds the `path` property to the module object. It contains the
current directory as path. That is necessary to add an extra caching
layer.

It also makes sure the `id` uses a default in case it's not set.
Otherwise the `path.dirname(id)` command could fail.

lib/internal/modules/cjs/loader.js

@@ -103,8 +103,9 @@ function updateChildren(parent, child, scan) {
     children.push(child);
 }
 
-function Module(id, parent) {
+function Module(id = '', parent) {
   this.id = id;
+  this.path = path.dirname(id);
   this.exports = {};
   this.parent = parent;
   updateChildren(parent, this, false);