src/Installer/PluginInstaller.php
public function getInstallPath(\Composer\Package\PackageInterface $package)
    {
        $packageType = $package->getType();

        $installDir = $this->initializeInstallDir($packageType);
        $installName = static::getInstallName($package, $this->rootPackage);
        return $installDir . '/' . $installName;
    }
public function getInstallPath(\Composer\Package\PackageInterface $package)
    {
        $packageType = $package->getType();

        $installDir = $this->initializeInstallDir($packageType);
        $installName = static::getInstallName($package, $this->rootPackage);
        return $installDir . '/' . $installName;
    }
Returns the installation path of a package

Plugins are installed to the `plugins/`, themes to the `themes/` dir
by default respectively. You can overwrite these target dirs using the
"pico-plugin-dir" resp. "pico-theme-dir" extra in the root package's
`composer.json`.

@param  \Composer\Package\PackageInterface $package
@return string