Favour non-preferred versions over deprecated versions

[jasagredo]

As of now, deprecated versions and non-preferred versions are treated equally: Cabal will find the preferred versions, try all of them, and if none is acceptable, then try the non-preferred and deprecated versions, with no difference among them.

My understanding is that

• non-preferred versions are perfectly functioning versions that are just less preferable than preferred versions
• deprecated versions are malfunctioning versions that should be avoided

So my proposed flow would be the following:

• when requested for package foo
• if there are preferred versions, try those in decreasing order of version number
• if none of those are acceptable, try the non-preferred and non-deprecated versions in decreasing order of version. If one is acceptable, pick it with a warning.
• if none of those are acceptable, try the deprecated versions in decreasing order of version. If one is acceptable, pick it with a warning/or don't even pick it unless some flag --allow-deprecated=[all|foo|...] is provided.

There are a bunch of issues on this matter, just mentioning deprecated and normal versions, but the non-preferred versions just complicate it even more:

• Warn about deprecated packages #1518
• Add a warning when a deprecated package is used in the build plan #9522
• Flag deprecated package versions in the solver rejection messages #9830
• A possibility to completely filter out deprecated packages. #8863

[grayjay]

I opened an issue for making this information available in the package index: haskell/hackage-server#1345