src/Composer/Installer/LibraryInstaller.php
protected function getPackageBasePath(PackageInterface $package)
    {
        $installPath = $this->getInstallPath($package);
        $targetDir = $package->getTargetDir();

        if ($targetDir) {
            return preg_replace('{/*'.str_replace('/', '/+', preg_quote($targetDir)).'/?$}', '', $installPath);
        }

        return $installPath;
    }
protected function getPackageBasePath(PackageInterface $package)
    {
        $installPath = $this->getInstallPath($package);
        $targetDir = $package->getTargetDir();

        if ($targetDir) {
            return preg_replace('{/*'.str_replace('/', '/+', preg_quote($targetDir)).'/?$}', '', $installPath);
        }

        return $installPath;
    }
Returns the base path of the package without target-dir path

It is used for BC as getInstallPath tends to be overridden by
installer plugins but not getPackageBasePath

@param  PackageInterface $package
@return string