Move REQUIREMENT_* constants out of install.inc file

Nowadays idea of discovery Requirement objects could use attributes to define title, value and description.
Other similar kind of checks/tasks install system has, so approach from #50 sounds viable

+++ b/core/lib/Drupal/Core/Render/Element/StatusReport.php
@@ -35,13 +37,13 @@ public static function preRenderGroupRequirements($element) {
     $severities = static::getSeverities();
...
-      $severity = $severities[REQUIREMENT_INFO];
+      $severity = $severities[Requirements::SEVERITY_INFO];
...
-        $severity = $severities[REQUIREMENT_OK];
+        $severity = $severities[Requirements::SEVERITY_OK];
...
       $grouped_requirements[$severity['status']]['title'] = $severity['title'];

+++ b/core/lib/Drupal/Core/Requirements.php
@@ -0,0 +1,30 @@
+final class Requirements {

it could be enum but it actually used only to group requirements

Seems like we have an issue that already has some work done in that way 📌 Add value objects to represent the return of hook_requirements Needs review

Looks yep!