Remove global variable from dir_status

[rgrinberg]

Now we store the dir_status database per super context. Similarly, we could remove the global cache in Dir_contents as well if I manage to make #1566 work.

[ghost]

FTR, I'm counting of #1489 to get rid of all these global variables. The idea is that getting the directory status will just become a memoized function and Dune will automatically track what computing the directory status requires. So we shouldn't bother getting rid of these global variables for now.

[rgrinberg]

The idea is that getting the directory status will just become a memoized function and Dune will automatically track what computing the directory status requires

Oh ok, I thought that getting rid of the global variables would help transitioning it to #1489. Since it now becomes what are the data dependencies of Dir_status.

[ghost]

It's possible, but I think the situation will be clearer once #1489 is merged and we can start refactoring the code.

[rgrinberg]

I'm re-opening because inverting the dep here is required for other changes. So unless this PR makes it harder to migrate to #1489, I'd like to see it merged, even if this code will be removed in the near future.