Better combining of metadatas

[hasufell]

Currently, we assume uniqueness of the tags recommended and latest.

This is mostly true when combining e.g. the default metadata with cross or with prereleases.

However, if a user wanted to combine ghcup-0.0.8.yaml with ghcup-vanilla-0.0.8.yaml, they might get duplicated tags, which will be confusing (the vanilla channel uses a different policy for recommended and might also be quicker with adding new versions, so latest could also diverge).

The use case for combining both these channels is that a user might want new releases as soon as possible, but wants to enjoy good bindists for recommended.

I'm still unsure on how to resolve the situation.