Cache more things in package management use-cases

[ElectreAAS]

We recently enabled the cache by default, although with a relatively small set of rules that can actually be cached. We'd like to cache as much as possible, especially regarding costly downloads and compilation of dependencies.

A few options have been discussed already (may or may not be mutually exclusive):

• Track down all instances where we can set the can_go_in_shared_cache to true
• Assuming we are building a package dependency, and that package gets built with a child dune: pass arguments to that child dune to cache everything
• Would work even without a child dune: cache everything when we come back to the parent dune

[ElectreAAS]

cc @leostera @Leonidas-from-XIV @rgrinberg

[rgrinberg]

Yeah, that sounds fine. You can experiment with always caching the OCaml rules as well. Those are defined within dune itself, so they should be pretty safe to cache.