Bug 67540 - Load extension CSS modules

This patch adds extensions' CSS modules to the list of modules
passed to load.php from Parsoid's <head> output.

The unconditional loading of genshi's CSS modules was removed
as they get loaded now only if the page actually uses them, as
they appear in the list of used modules retrieved from the page.
This was tested both by looking at the produced load.php
parameters and by using parsoid_visual_diff to ensure the
rendered output is the same.

TODO:
* Load other module parts like JS, messages, etc.
* Get module information for parser functions and support those
too, possibly by adding a props=modules option to
action=expantemplates similar to the one in action=parse.

Change-Id: I2b96a96a296919920c1c13cc34e78c603a0faea2