MDL-81960 core: Update namespace of progress_trace classes

lib/classes/output/progress_trace/html_progress_trace.php

@@ -14,6 +14,10 @@
 // You should have received a copy of the GNU General Public License
 // along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
 
+namespace core\output\progress_trace;
+
+use core\output\progress_trace;
+
 /**
  * This subclass of progress_trace outputs as HTML.
  *
@@ -31,3 +35,8 @@ class html_progress_trace extends progress_trace {
         flush();
     }
 }
+
+// Alias this class to the old name.
+// This file will be autoloaded by the legacyclasses autoload system.
+// In future all uses of this class will be corrected and the legacy references will be removed.
+class_alias(html_progress_trace::class, \html_progress_trace::class);