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!

I am going to work on this because we need these constants for hook_update_requirements and hook_runtime_requirements.

The one downside is this approach is only available to core and not core modules/themes, or contrib or custom because they don't using composer to set up their autoloading. But I'm not sure that outweighs the advantages listed.

Since these classes are used in core, contrib, and custom, I think this disqualifies the constants.php approach.

However, we don't actually have to deprecate these here, I think we can do that when we deprecate hook_requirements, for now we can just create the new classes and just use them as we convert hooks.

Deprecation suppose removal of the usage, 240LOC surely needs chunking

git grep REQUIREMENT_ |wc -l
241

Yes, I wouldn't create the deprecation here, just create the new landing place for constants and the new constants we need.

I wonder if there is a way to have both.

E,g. something like
/Drupal/Core/Constants
That is where the constants live, you can use it if you need it.
Maybe it makes sense to auto load it so they are available everywhere that uses autoloading.

I am not going to work on this, I think @berdir found a clean workaround, I think my comments about constants.php are still valid.