CSSResourceLocator: account for symlinks in app path

Currently, if the app path includes a symlink, the calculated webDir
will be incorrect when generating SCSS and CSS will break.

Use realpath when retrieving app path, and these issues go away.

Fix #6028

Signed-off-by: Kyle Fazzari <kyrofa@ubuntu.com>

lib/private/Template/CSSResourceLocator.php

@@ -71,6 +71,11 @@ public function doFind($style) {
 			return;
 		}
 
+		// Account for the possibility of having symlinks in app path. Doing
+		// this here instead of above as an empty argument to realpath gets
+		// turned into cwd.
+		$app_path = realpath($app_path);
+
 		if(!$this->cacheAndAppendScssIfExist($app_path, $style.'.scss', $app)) {
 			$this->append($app_path, $style.'.css', $app_url);
 		}