fix(pkg): dune show depexts prints on stdout

[Alizter]

• pkg changelog
• Addresses some concerns in Print output of some commands to stdout instead of stderr #12586

cc @punchagan @Leonidas-from-XIV

[punchagan]

On a side note, should pkg search be doing something similar?

[Alizter]

@punchagan That may make sense. I think the main benefit of doing it for dune show depexts is that it will be composed with other commands, for which stdout is more convenient. Do you think that will be the case with dune pkg search?

[Leonidas-from-XIV]

I definitely see this less with dune pkg search (however dune pkg search | grep REGEXP is something I could definitely see happening) but in general I think we should have a way to output to stdout.

Stdout is meant for user-requested output (like dune show) and stderr is for other information (e.g. warnings and I guess progress information), so I think going through the commands and changing where they are written to is probably a decent UX improvement.