give full explanation of recompilation reason

[Khady]

↻ recompile ocurl                           20231103+git
          [upstream or system changes]

Once in a while during an opam upgrade or opam install opam gives us a message like this, telling us that a package will be recompiled. But it doesn't give the true reason why the recompilation happens, even though opam exactly knows why. It makes debugging much more troublesome that it should by hiding useful information behind a message that can't be leveraged at all.

[Khady]

00:00.597  STATE                  Detected changed packages (marked for reinstall): { ocurl.20231103+git }

Even with debug and verbose modes enabled, the amount of details is way too low

[kit-ty-kate]

That sounds like a good idea (though it probably requires a consequential change internally and to think seriously about the UI).
As a side note, note that since #5118 (>= opam.2.2.0~alpha1), only required changes trigger a recompile so you should encounter them a lot less.