Move 'can_go_in_shared_cache' from Action to Rule, with the same default

[ElectreAAS]

This is my second attempt at going in the direction of caching by default, and caching downloads, following the discussion over at #10729.
I realized that the field can_go_in_shared_cache in Dune_Engine.Action.Full.t already existed, had a default to true, and was never set to false anywhere. I then hijack this by moving the field to Rule.t instead. This makes it so that all calls to Rule.make can now set this variable, and the goal is to change the call in pkg_rule to true when the PR is ready.

cc @Leonidas-from-XIV

[ElectreAAS]

TODO comment intentionnaly left here, as removing the true in the tuple changes the digest, and trips the tests. I wanted to make a separate commit that only removes this line and changes the hash to make it clean for the CI.

[rgrinberg]

How does it help you to move the flag from the rule to the action? Every rule has a corresponding action, so I think you can just set the flag on the action in the rule.

[ElectreAAS]

How does it help you to move the flag from the rule to the action? Every rule has a corresponding action, so I think you can just set the flag on the action in the rule.

As you can probably guess I'm a little bit confused by the situation.
The field can_go_in_shared_cache in Action.t sounds like it should do the trick, and I can just set this flag to true in more places than currently. However, it is set by default to true, and I can't find a place in the code where that default value is not used. Is this field really doing the thing it should?
Moreover, if I want to enable caching package downloads, can I trust that by adding a variable in dune_rules/pkg_rules it's going to make its way to dune_engine/build_system? That file is the one place where the field is actually used, but I don't know if package rules actually make it there...

[rgrinberg]

You're on the right track. To recap:

1. We want to enable the dune cache by default
2. But doing so will break a lot of existing code that might not be cacheable
3. So we flip the default, but set this flag for all rules to say that they're incompatible with the dune cache.
4. We make an exception for the rules defined by pkg_rules.ml. Those should now be explicitly compatible.
5. But flipping 3, we've made it impossible for users to get back the old behavior of caching everything. So we can introduce a dune flag, say DUNE_CACHE_RULE_DEFAULT=false|true to allow user to get back to the old world where everything would be cached.

Hope that makes sense. Feel free to ask more.

[ElectreAAS]

Thanks a lot for the explanation! I think I made some progress back on #10710.