extensionSystem: Store extensions in a Map

After making the extensions map private to the ExtensionManager, we can
switch it to a proper hash table which is more appropriate.

https://bugzilla.gnome.org/show_bug.cgi?id=789852

js/misc/extensionUtils.js

@@ -70,7 +70,7 @@ function getCurrentExtension() {
     // Walk up the directory tree, looking for an extension with
     // the same UUID as a directory name.
     while (file != null) {
-        let extension = extensionManager.extensions[file.get_basename()];
+        let extension = extensionManager.lookup(file.get_basename());
         if (extension !== undefined)
             return extension;
         file = file.get_parent();