Cacheability is not respected when not using a lazy builder

Did some digging, and found this in FieldFormatterBase::view():

    // Field item lists, in particular for computed fields, may carry cacheable
    // metadata which must be bubbled.
    if ($items instanceof CacheableDependencyInterface) {
      (new CacheableMetadata())
        ->addCacheableDependency($items)
        ->applyTo($elements);
    }

So it looks as though ComputedFieldClass needs to implement CacheableDependencyInterface and pass on the cacheable metadata from the plugin. Patch coming up.

First attempt at seeing how we might solve this. Needs work for sure, but wanted to get feedback on the approach.

> Did some digging, and found this in FieldFormatterBase::view():

Nice find!

I've fixed the broken tests -- could you rebase this please so the tests can run on it again?

Rebased!

Thanks!

I started looking at this, and figured I'd add test coverage. I've got something working (in branch 3487785-cacheability-tests) -- but I need to do a bit of clean-up on it as I had to try several approaches to get caching to kick in!

Hmm on second thoughts, I can't get it to work :/
Pushed everything to the branch.