Ensure packages in additional installed repository are not updated

The underlying assumption of the additional installed repository is that those
packages are installed in such a way that they cannot be updated. Prior to this
fix they were still candidates for change. This ensures that they are treated
like platform and root packages and are not candidates for update.

src/Composer/Installer.php

@@ -560,8 +560,14 @@ class Installer
         $constraint->setPrettyString($rootPackage->getPrettyVersion());
         $request->install($rootPackage->getName(), $constraint);
 
+        $installedPackages = $platformRepo->getPackages();
+        if ($this->additionalInstalledRepository) {
+            $additionalInstalledPackages = $this->additionalInstalledRepository->getPackages();
+            $installedPackages = array_merge($installedPackages, $additionalInstalledPackages);
+        }
+
         // fix the version of all platform packages to prevent the solver trying to remove those
-        foreach ($platformRepo->getPackages() as $package) {
+        foreach ($installedPackages as $package) {
             $constraint = new VersionConstraint('=', $package->getVersion());
             $constraint->setPrettyString($package->getPrettyVersion());