src/Composer/Installer.php
private function mockLocalRepositories(RepositoryManager $rm)
    {
        $packages = array();
        foreach ($rm->getLocalRepository()->getPackages() as $package) {
            $packages[(string) $package] = clone $package;
        }
        foreach ($packages as $key => $package) {
            if ($package instanceof AliasPackage) {
                $alias = (string) $package->getAliasOf();
                $packages[$key] = new AliasPackage($packages[$alias], $package->getVersion(), $package->getPrettyVersion());
            }
        }
        $rm->setLocalRepository(
            new InstalledArrayRepository($packages)
        );
    }
private function mockLocalRepositories(RepositoryManager $rm)
    {
        $packages = array();
        foreach ($rm->getLocalRepository()->getPackages() as $package) {
            $packages[(string) $package] = clone $package;
        }
        foreach ($packages as $key => $package) {
            if ($package instanceof AliasPackage) {
                $alias = (string) $package->getAliasOf();
                $packages[$key] = new AliasPackage($packages[$alias], $package->getVersion(), $package->getPrettyVersion());
            }
        }
        $rm->setLocalRepository(
            new InstalledArrayRepository($packages)
        );
    }
Replace local repositories with InstalledArrayRepository instances

This is to prevent any accidental modification of the existing repos on disk

@param RepositoryManager $rm