v2.0.x does not support multiple licenses definition

Hello,

I've noticed that if we have a project with multiple licenses (as explained in Composer documentation, we cannot build a manifest.xml file that reflect the project situation.

See :

• https://github.com/phar-io/manifest/blob/master/manifest.xsd#L43
• https://github.com/phar-io/manifest/blob/2.0.3/src/xml/CopyrightElement.php#L19