Optimize package registration (#30960)

As per the discussion in #30952 and according to my tests, `strpos` is considerably faster than calling startsWith for short haystacks.

src/Illuminate/Foundation/Application.php

@@ -584,7 +584,7 @@ public function registerConfiguredProviders()
     {
         $providers = Collection::make($this->config['app.providers'])
                         ->partition(function ($provider) {
-                            return Str::startsWith($provider, 'Illuminate\\');
+                            return strpos($provider, 'Illuminate\\') === 0;
                         });
 
         $providers->splice(1, 0, [$this->make(PackageManifest::class)->providers()]);