Set explicit visibility on class constants

src/MDExternalHostingSet.php

@@ -11,9 +11,9 @@
  */
 final class MDExternalHostingSet extends MDValueSet {
 
-    const DEFAULT_EXTERNAL_HOSTING = 'Archive.org';
+    public const DEFAULT_EXTERNAL_HOSTING = 'Archive.org';
 
-    const EXTERNAL_SOURCES = [
+    public const EXTERNAL_SOURCES = [
         'Archive.org',
     ];