bug #25258 [link] Prevent warnings when running link with 2.7 (dunglas)

This PR was merged into the 2.7 branch.

Discussion
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[link] Prevent warnings when running link with 2.7

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | yes
| New feature?  | no <!-- don't forget to update src/**/CHANGELOG.md files -->
| BC breaks?    | no
| Deprecations? | no <!-- don't forget to update UPGRADE-*.md files -->
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Prevent warnings when linking from 2.7 because some directories (like the Bridge/Propel1) don't contain `composer.json` files.

Commits
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3e7780c [link] Prevent warnings when running link with 2.7

link

@@ -35,11 +35,14 @@ if (!is_dir("$argv[1]/vendor/symfony")) {
 }
 
 $sfPackages = array('symfony/symfony' => __DIR__);
+
+$filesystem = new Filesystem();
 foreach (glob(__DIR__.'/src/Symfony/{Bundle,Bridge,Component,Component/Security}/*', GLOB_BRACE | GLOB_ONLYDIR | GLOB_NOSORT) as $dir) {
-    $sfPackages[json_decode(file_get_contents("$dir/composer.json"))->name] = $dir;
+    if ($filesystem->exists($composer = "$dir/composer.json")) {
+        $sfPackages[json_decode(file_get_contents($composer))->name] = $dir;
+    }
 }
 
-$filesystem = new Filesystem();
 foreach (glob("$argv[1]/vendor/symfony/*", GLOB_ONLYDIR | GLOB_NOSORT) as $dir) {
     $package = 'symfony/'.basename($dir);
     if (is_link($dir)) {