Add support for retrieving raw file content from GitLab

[gavanderhoorn]

As per title.

I had a couple packages which were hosted on gitlab.com but were not released / did not have a release repository on github.com.

Adding them to rosdistro_additional_recipes.yaml works fine, but when Vinca tried to download their manifests it ran into this line and refused.

GitLab raw URLs are relatively simple, and c6092b4 adds a formatter for them. Follow-up commits then use it when needed.

Checking for host substrings in URLs is a bit brittle, but I did not want to diverge from the existing pattern too much.

The GitHub download path should not be affected by the refactor, it's mostly just shuffling existing code around.

[gavanderhoorn]

I'm also wondering how to support packages not on github.com or gitlab.com, but on self-hosted GitLab instances fi.

The latter could use _construct_raw_url_gitlab(..), but without a domain to match against I'm not sure how Vinca would detect which variant of the formatter it should use.