MDL-58285 tool_mobile: Web service generates incorrect fileurl on IIS

The tool_mobile_get_plugins_supporting_mobile generated an incorrect
fileurl on IIS. The /mobile/filename.zip part was generated as
\mobile\filename.zip, this failed the PARAM_URL check.

This change stops the use of OS specific directory separators and uses
the linux/web separator. PHP file functions recognise this properly.

admin/tool/mobile/classes/api.php

@@ -69,7 +69,7 @@ class api {
                     );
 
                     // All the mobile packages must be under the plugin mobile directory.
-                    $package = $path . DIRECTORY_SEPARATOR . 'mobile' . DIRECTORY_SEPARATOR . $addonname . '.zip';
+                    $package = $path . '/mobile/' . $addonname . '.zip';
                     if (file_exists($package)) {
                         $plugininfo['fileurl'] = $CFG->wwwroot . '' . str_replace($CFG->dirroot, '', $package);
                         $plugininfo['filehash'] = sha1_file($package);