mediawiki.page.gallery.styles: Move files to src/

The two CSS files belonging to this module are the last ones
remaining in src/mediawiki/page. Move them to their own directory
in src/ instead. The stylesheets don't reference any images.

Bug: T193826
Change-Id: Ic07bbd5b60668a437177b389aa8fee861eb11892

resources/Resources.php

@@ -1687,8 +1687,8 @@ return [
 	],
 	'mediawiki.page.gallery.styles' => [
 		'styles' => [
-			'resources/src/mediawiki/page/gallery.css',
-			'resources/src/mediawiki/page/gallery.print.css' => [ 'media' => 'print' ],
+			'resources/src/mediawiki.page.gallery.styles/gallery.css',
+			'resources/src/mediawiki.page.gallery.styles/print.css' => [ 'media' => 'print' ],
 		],
 		'targets' => [ 'desktop', 'mobile' ],
 	],