Modify the GitLab importer to support package-first mode

[michaelehab]

As a part of the ongoing GSoC 2025 project titled "On-demand live evaluation of packages and Integration with VulnTotal", this issue should set the base for using the new API endpoint.
It should modify the gitlab importer to support the package-first mode.
After the implementation of this issue, when a user calls the new API endpoint with Gitlab, the advisories database should only have those related to the input PackageURL.