Add forward compatibility for upcoming v2 installed.json format, refs #7999

src/Composer/Repository/FilesystemRepository.php

@@ -51,6 +51,11 @@ class FilesystemRepository extends WritableArrayRepository
         try {
             $packages = $this->file->read();
 
+            // forward compatibility for composer v2 installed.json
+            if (isset($packages['packages'])) {
+                $packages = $packages['packages'];
+            }
+
             if (!is_array($packages)) {
                 throw new \UnexpectedValueException('Could not parse package list from the repository');
             }