module: inherit unique lookup paths from parent

The previous commit added the module lookup paths based on a symlink
location as well as the realpath location.  However, _subsequent_
modules loaded in that environment would _not_ have those additional
paths added to their lookup path, since they are not loaded based on a
linked location.

This patch adds any paths from the parent module lookup paths to the
child, if the child does not already have that path in their lookup
list.

lib/module.js

@@ -513,6 +513,13 @@ Module.prototype.load = function(realpath, filename) {
   const realdir = path.dirname(realpath);
   const linkdir = realpath !== filename ? path.dirname(filename) : null;
   this.paths = Module._nodeModulePaths(realdir, linkdir);
+  if (this.parent && this.parent.paths) {
+    for (let i = 0; i < this.parent.paths.length; i++) {
+      if (this.paths.indexOf(this.parent.paths[i]) === -1) {
+        this.paths.push(this.parent.paths[i]);
+      }
+    }
+  }
 
   var extension = path.extname(filename) || '.js';
   if (!Module._extensions[extension]) extension = '.js';