reduce/simplify revdeps checks by never using `avoid-version` packages

[raphael-proust]

The CI run https://opam.ci.ocaml.org/github/ocaml/opam-repository/commit/e4a40d9efaf0f57348f6edf811dc5513cf04e4c6 fails some revdeps tests. One of these tests is for cohttp-curl-async.6.0.0~alpha1 which is marked as flags: [ avoid-version ] in the package repository.

I don't think that the revdeps should use avoid-version packages.

I'd even go further and say that revdeps tests should never use any package marked as avoid-version, even as a dep to a package they are testing. But idk if that's easy to set-up.