MDL-59969 admin: Warn admins if a development libs directory exists

We can't really control the direct web access to directories in dirroot,
that is part of the server setup. So we at least warn admins as they may
not realize the risks of having directories like vendor or node_modules
exposed.

Credit goes to Petr Škoda for mentioning the PHPUnit issue CVE-2017-9841
to me.

admin/renderer.php

@@ -276,12 +276,15 @@ class core_admin_renderer extends plugin_renderer_base {
      * @param int|null $availableupdatesfetch timestamp of the most recent updates fetch or null (unknown)
      * @param string[] $cachewarnings An array containing warnings from the Cache API.
      * @param array $eventshandlers Events 1 API handlers.
+     * @param bool $themedesignermode Warn about the theme designer mode.
+     * @param bool $devlibdir Warn about development libs directory presence.
      *
      * @return string HTML to output.
      */
     public function admin_notifications_page($maturity, $insecuredataroot, $errorsdisplayed,
             $cronoverdue, $dbproblems, $maintenancemode, $availableupdates, $availableupdatesfetch,
-            $buggyiconvnomb, $registered, array $cachewarnings = array(), $eventshandlers = 0, $themedesignermode = false) {
+            $buggyiconvnomb, $registered, array $cachewarnings = array(), $eventshandlers = 0,
+            $themedesignermode = false, $devlibdir = false) {
         global $CFG;
         $output = '';
 
@@ -290,6 +293,7 @@ class core_admin_renderer extends plugin_renderer_base {
         $output .= $this->legacy_log_store_writing_error();
         $output .= empty($CFG->disableupdatenotifications) ? $this->available_updates($availableupdates, $availableupdatesfetch) : '';
         $output .= $this->insecure_dataroot_warning($insecuredataroot);
+        $output .= $this->development_libs_directories_warning($devlibdir);
         $output .= $this->themedesignermode_warning($themedesignermode);
         $output .= $this->display_errors_warning($errorsdisplayed);
         $output .= $this->buggy_iconv_warning($buggyiconvnomb);
@@ -520,6 +524,24 @@ class core_admin_renderer extends plugin_renderer_base {
         }
     }
 
+    /**
+     * Render a warning that a directory with development libs is present.
+     *
+     * @param bool $devlibdir True if the warning should be displayed.
+     * @return string
+     */
+    protected function development_libs_directories_warning($devlibdir) {
+
+        if ($devlibdir) {
+            $moreinfo = new moodle_url('/report/security/index.php');
+            $warning = get_string('devlibdirpresent', 'core_admin', ['moreinfourl' => $moreinfo->out()]);
+            return $this->warning($warning, 'error');
+
+        } else {
+            return '';
+        }
+    }
+
     /**
      * Render an appropriate message if dataroot is insecure.
      * @param bool $errorsdisplayed