Add support for single scripts, styles and debugScripts

Objective:

* Make makeStaticLoader.php compatible with ResourceLoader module
  definitions that use a string instead of an array with a single element
  when describing a single file

Changes:

maintenance/makeStaticLoader.php
* Cast styles, scripts and debugScripts to array when iterating over them

Change-Id: Ib91daba732c2035145c479467a0b6b9ca3155659
Merge "Add support for single scripts, styles and debugScripts"