Repro case for loading too many rules for @fmt

[Leonidas-from-XIV]

When doing dune build @fmt Dune loads the rules and, given the package does not exist fails to evaluate the pform. However in the case of cram stanzas, there's no need to evaluate these pforms as the result should not matter for formatting.

Originally reported by @jonludlam in ocaml/odoc#1490

[Alizter]

Maybe what's needed here is some sort of %{pkg-available:..} so it can properly guard the version check?

[Leonidas-from-XIV]

Maybe. I was also wondering whether it would make sense to coerce evaluation failures in if_available to false, though that has a bit of a risk to hide issues with typos and thus never trigger at all.

[Alizter]

The issue with the cram stanza is that we need to interpret the enabled_if as early as possible. It can potentially contribute to the fmt alias by using the aliases field for instance. Other than guarding enabled_if with some proper thing I don't see how we could support this.

I'll note that I haven't thought through what pkg-available will entail, and so its just an idea. In the meantime, you could get away with bin-available or lib-available from the package in question to guard the expression.