Detect and filter out deprecated packages

[kit-ty-kate]

We need some way of tagging packages as "deprecated" so that they don't show up when the "Only show the latest version of each packages" option is enabled.

Would a deprecated flag in the opam file work? e.g.

flags: [deprecated]

[avsm]

I think that's worth an upstream ocaml/opam issue, to help spec it out, as this is useful beyond health check. There have been requests for this for marking older packages that have been renamed (e.g. conduit-mirage <- mirage-conduit) or for ones the author doesn't want to support any more for their own reasons.

For these sorts of features, we could start with an x-field, but then move to deeper integration with the solver. I'd imagine an initial version could have the 0install-solver mark any packages with an x-deprecated as conflicts to the solver so they are not selected in dependencies either.

When we add fields to opam in a future revision, we could have a deprecated or alerts field to complement the current available.

[kit-ty-kate]

I think I'd rather have deprecated flag than a x-deprecated field. A x-deprecated field doesn't make much sense to me as I do not see what the value would be. flags: deprecated passes the opam lint test so there is no harm in adding it.

I'll open an issue for a solver integration though, for sure.