Broken url for dependencies managed by CTAO (or related) and not hosted on CTAO gitlab

Summary

The url creation just adds https://gitlab.cta-observatory.org/ prefix. This will be broken, if a full url is provided to gitlab_path var. Suggestions

• add source_url variable, make it mutually exclusive with the gitlab_path to preserve API compatibility
• parse full path from either of these variables.

/cc @mlinhoff