MDL-65995 components: Too early to use debugging()

components are loaded very early in Moodle bootstrap/warmup
process. So, basically, core_component must be vanilla php
and moodlelib/weblib facilitites are not available yet.

So, changing debugging() call to error_log(), to make it
consistent with the rest of the calls in the method.

lib/classes/component.php

@@ -537,9 +537,8 @@ $cache = '.var_export($cache, true).';
         if (file_exists("$ownerdir/db/subplugins.json")) {
             $subplugins = (array) json_decode(file_get_contents("$ownerdir/db/subplugins.json"))->plugintypes;
         } else if (file_exists("$ownerdir/db/subplugins.php")) {
-            debugging('Use of subplugins.php has been deprecated. ' .
-                    'Please update your plugin to provide a subplugins.json file instead.',
-                    DEBUG_DEVELOPER);
+            error_log('Use of subplugins.php has been deprecated. ' .
+                "Please update your '$ownerdir' plugin to provide a subplugins.json file instead.");
             include("$ownerdir/db/subplugins.php");
         }