resourceloader: Make 'site.styles' actually in group 'site'

resourceloader: Make 'site.styles' actually in group 'site'

Before 93ed259cf and 7bb7315d4, 'site' and 'site.styles' module were one module
loaded in two parts. One part via the general queue (not in a separate request),
and the other part in the stylesheet queue (in a separate request).

This was achieved by hacking the stylesheet queue handler to hardcode the group
to be 'site' for a module named 'site' (later 'site.styles' in c20a76ee)
eventhough the module as a whole was not actually in that group.

Now that these modules are separated, this hack is no longer needed
and the module can be registered as being in group 'site' properly.

Change-Id: If193f556be211749b115f3808ee46536b578fb22