src/Installer/Package/ModulePackageInstaller.php
protected function formSourcePath($packagePath)
    {
        $sourceDirectory = $this->getExtraParameterValueByKey(static::EXTRA_PARAMETER_KEY_SOURCE);

        return !empty($sourceDirectory)?
            Path::join($packagePath, $sourceDirectory):
            $packagePath;
    }
protected function formSourcePath($packagePath)
    {
        $sourceDirectory = $this->getExtraParameterValueByKey(static::EXTRA_PARAMETER_KEY_SOURCE);

        return !empty($sourceDirectory)?
            Path::join($packagePath, $sourceDirectory):
            $packagePath;
    }
If module source directory option provided add it's relative path.
Otherwise return plain package path.

@param string $packagePath

@return string