ResourceLoaderFileModule: Implement remoteSkinPath option

remoteSkinPath works the same as remoteExtPath but is relative to skins/ instead of extensions/

This will allow skins to register modules using:
 $wgResourceModules['skin.myskin'] = array(
   // ...
   'localBasePath' => __DIR__,
   'remoteSkinPath' => 'myskin',
 );

Instead of using:
 'remoteBasePath' => $GLOBALS['wgStylePath'] . '/myskin', // or
 'remoteBasePath' => &$GLOBALS['wgStylePath'],

Change-Id: I0e8c4a37a224e9528c9c5aa5417f0f56dbb89b97
Merge "ResourceLoaderFileModule: Implement remoteSkinPath option"