Show error message when dune-workspace disables package management

[Leonidas-from-XIV]

Currently you can have no global dune config file but your workspace has (pkg disabled). In this case the code currently assumes that writing (pkg enabled) is enough to enable package management but when actually trying to build package management is disabled and there is no ocamlc so the build will fail.

This PR adds an error message (unfortunately folded by default) in the case where the workspace disables package management, so we don't attempt to build with disabled package management.

[shym]

I didn’t understand by reading your commit messages and your PR message why you get rid of dune_aux? (It is not relevant to show traces when dune pkg enabled exits on failure, as it’s an intended behaviour, I admit. If that’s indeed what you had in mind (and that I didn’t miss something else), I’ll propose an update to your PR for that)

[Leonidas-from-XIV]

The issue I had was that dune_aux does not seem to properly return non-zero exit codes. If I call dune_aux pkg enabled and the code is 1 In my experience it will terminate and never return the 1. I spent a lot of time pulling out my hair why the CI would always terminate instead if displaying my dune_aux pkg enabled || alert "oops" failure.

In the end I just skipped dune_aux because it did a number of things that aren't necessary. That said, I had to add the workspace option back in and its still not working properly as can be seen from the test failures, so probably the better option is to fix dune_aux. Will look into that.

[shym]

Thanks for your explanations, I’ve updated the PR to fix the issues you encountered and rebase your change on top.
What do you think?

[Leonidas-from-XIV]

Yes, I think this is a nicer solution. It also resolves the issue with the tests in my previous version.