opam show ocaml should not show unreleased versions

[kit-ty-kate]

Similarly to #6098, it is surprising for users to see opam show ocaml display 5.4.0 (unreleased at the time of writing and only available via the ocaml-variants.5.4.0+trunk package which is tagged with avoid-version) as the latest available version (all-versions field).

To remedy that, i propose to hide unavailable (currently displayed red and crossed, if the terminal allows it) as well as versions only available using avoid-version/deprecated dependencies by default from the all-versions field unless --all is given.

[dra27]

Not sure how this could be done in general without involving the solver (which seems a bit heavy for opam show?) - but specifically for the ocaml package, we could actually flag ocaml.5.4.0 as avoid-version until either ocaml-base-compiler.5.4.0~alpha is released or even the full release itself?

[kit-ty-kate]

For some reason i hadn't thought of the cost of checking and it does indeed involve the solver in some capacity.

I agree with your proposal and i've opened ocaml/opam-repository#27227 and #6358 to show what it would look like.