opam switch list-available should only display packages that aren't flagged with avoid-version/deprecated

[kit-ty-kate]

https://discuss.ocaml.org/t/error-on-opam-install-utop-fedora/14948/6

I think that command could be improved by only returning the list of packages without those flags by default and adding a --all argument to get the previous behaviour

[dra27]

For the UI (as in opam switch list-available being used at the console by the user, rather than for a script) we'll want some kind of indication that there are avoid-version packages which aren't listed. A little like the warning on opam upgrade when some upgrades are unavailable.