MDL-43530 cache: Cache multiple different dirroots

In some clustered environments (Amazon OpsWorks) the deployment
folders on different nodes use different file paths.  As we cache
the realpath in the shared MUC, you don't always get the right answer.

We now cache each dirroot in it's own full path location to allow
each node to get the correct file locations for itself.

lib/moodlelib.php

@@ -8227,14 +8227,9 @@ function get_plugin_types($fullpaths=true) {
     $cache = cache::make('core', 'plugintypes');
 
     if ($fullpaths) {
-        // First confirm that dirroot and the stored dirroot match.
-        if ($CFG->dirroot === $cache->get('dirroot')) {
-            // They match we can use it.
-            $cached = $cache->get(1);
-        } else {
-            // Oops they didn't match. The moodle directory has been moved on us.
-            $cached = false;
-        }
+        // Cache each dirroot separately in case cluster nodes happen to be deployed to
+        // different locations.
+        $cached = $cache->get(sha1($CFG->dirroot));
     } else {
         $cached = $cache->get(0);
     }
@@ -8293,10 +8288,7 @@ function get_plugin_types($fullpaths=true) {
         }
 
         $cache->set(0, $info);
-        $cache->set(1, $fullinfo);
-        // We cache the dirroot as well so that we can compare it when we
-        // retrieve full info from the cache.
-        $cache->set('dirroot', $CFG->dirroot);
+        $cache->set(sha1($CFG->dirroot), $fullinfo);
 
         return ($fullpaths ? $fullinfo : $info);
     }