resourceloader: Allow skins to provide additional styles for any module

The newly introduced $wgResourceModuleSkinStyles global enables skins to
provide additional stylesheets to existing ResourceLoader module.

This both makes it easier (or at all possible) to override default
styles and lowers the style footprint by making it possible not to
load styles unused on most pages.

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Example:

Use the file 'foo-styles.css' for the 'mediawiki.foo' module when using
the MySkin skin:

  $wgResourceModuleSkinStyles['myskin'] = array(
    'mediawiki.foo' => 'foo-styles.css',
    'remoteSkinPath' => 'MySkin',
    'localBasePath' => __DIR__,
  );

For detailed documentation, see the doc comment in DefaultSettings.php.
For a practical usage example, see Vector.php.

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Implementation notes:

* The values defined in $wgResourceModuleSkinStyles are embedded into
  the modules as late as possible (in ResourceLoader::register()).
* Only plain file modules are supported, setting module skin styles
  for other module types has no effect.
* ResourceLoader and ResourceLoaderFileModule now support loading
  files from arbitrary paths to make this possible, defined using
  ResourceLoaderFilePath objects.
  * This required some adjustments in seemingly unrelated places for
    code which didn't handle the paths fully correctly before.
* ResourceLoader and ResourceLoaderFileModule are now a bit more
  tightly coupled than before :(
* Included a tiny example change for the Vector skin, a lot more of
  similar cleanup is possible and planned for the future.
* Many of the non-essential mediawiki.* modules defined in
  Resources.php should be using `'skinStyles' => array( 'default' => … )`
  instead of `'styles' => …` to allow more customizations, this is
  also planned for the future after auditing which ones would actually
  benefit from this.

Change-Id: Ica4ff9696b490e35f60288d7ce1295766c427e87