We should use fromager's GitLabTagProvider to automatically turn git repo tags to version numbers for resolving which versions of vllm can build. We should retain the constraints.txt entries to let each variant build a different version, until we can automatically build all new versions.

It looks like the pattern is to have a midstream/downstream tag that adds an extra digit to the version number (0.8.3 is tagged 0.8.3.0_downstream, for example).

There is a mix of patterns, like v0.8.4.2_downstream, 0.8.3.0_downstream, midstream-v0.8.5.0, midstream-room-v0.9.0.1.0 today. That last one makes it seem like we need a variant-specific regex, at least to handle some of the cases.