Vector: Don't create an extra request for skins.vector module

Blank MediaWiki page:

head>
 load.php:startup
 load([ top ])
body>
 load.php:skins.vector&only=scripts
 load([ bottom ])

Skin styles are loaded outside ResourceLoader from the <head>
(so they don't wait for startup, and don't rely on javascript).
Fair enough for basic page styling.

Then the real `mw.loader`-able part of the module should be
separated. There is no reason for it to also be loaded outside
`mw.loader`.

Change-Id: Ife69c5a443aac9eec8c2ec06d986b53b2379816f
Merge "Vector: Don't create an extra request for skins.vector module"