[Config] Do not use absolute path when computing the vendor freshness

When one uses Docker with a different mounting point between CLI & FPM,
the cache keeps regenerating because the ComposerResource class see a
different path for each SAPI. For example `/home/app/app/vendor` vs `/var/www/app/vendor`.
So if you hit FPM, then the CLI, then FPM, each time a new cache is
generated. So the application is quite slow in dev env. And for people
on MacOSX (with docker) is a big pain! And obvisouly, this never
stabilizes !

This occurs a lot when you have a worker, that crash and reboot in the
background, and you browse the web interface. Or when you have something
that hit your API every X secondes, and you are working on a worker.

src/Symfony/Component/Config/Resource/ComposerResource.php

@@ -71,12 +71,14 @@ private static function refresh()
     {
         self::$runtimeVendors = [];
 
+        $i = 0;
         foreach (get_declared_classes() as $class) {
             if ('C' === $class[0] && 0 === strpos($class, 'ComposerAutoloaderInit')) {
                 $r = new \ReflectionClass($class);
                 $v = \dirname(\dirname($r->getFileName()));
                 if (file_exists($v.'/composer/installed.json')) {
-                    self::$runtimeVendors[$v] = @filemtime($v.'/composer/installed.json');
+                    self::$runtimeVendors["__vendor_$i"] = @filemtime($v.'/composer/installed.json');
+                    ++$i;
                 }
             }
         }

src/Symfony/Component/Config/Tests/Resource/ComposerResourceTest.php

@@ -21,11 +21,10 @@ public function testGetVendor()
     {
         $res = new ComposerResource();
 
-        $r = new \ReflectionClass(ClassLoader::class);
         $found = false;
 
         foreach ($res->getVendors() as $vendor) {
-            if ($vendor && 0 === strpos($r->getFileName(), $vendor)) {
+            if ($vendor && 0 === strpos('__vendor_0', $vendor)) {
                 $found = true;
                 break;
             }