Update ComposerPlugin.php

On Windows systems where the project directory is a junction or symlink, realpath() must be called on any directories (withing a subtree) you wish to compare relative paths for, because otherwise the directory upon which realpath() was called will point to the link target (original) and the other will point to the link (copy) which may be on a different filesystem (resulting in an invalid projectDir path like `C:\project\\Z:\project\` in the autoload_runtime.php)

Internal/ComposerPlugin.php

@@ -58,7 +58,7 @@ public function uninstall(Composer $composer, IOInterface $io): void
 
     public function updateAutoloadFile(): void
     {
-        $vendorDir = $this->composer->getConfig()->get('vendor-dir');
+        $vendorDir = realpath($this->composer->getConfig()->get('vendor-dir'));
 
         if (!is_file($autoloadFile = $vendorDir.'/autoload.php')
             || false === $extra = $this->composer->getPackage()->getExtra()['runtime'] ?? []