[WIP] update/bugfix to Export.php that enables exporting select fields that reference multiple other contenttypes

[simongroenewolt]

This PR updates the Export.php code so it uses a different way to look up (select) field content if it references other contenttypes. This was needed for my project to correctly export select fields that reference multiple content types. The current version would not export any valid data for my select fields.

See #7972

I don't know if Bolt 3.7 is still being updated for this kind of stuff, and I don't have any experience using 3.7 myself so testing of this fix was limited to my current project only.

Labeled WIP just to warn this is not thoroughly tested and should not just be merged. Waiting for some discussion before marking it as a 'real' PR.

[bobdenotter]

Thanks for the PR, @simongroenewolt.

The change looks good at first glance, but i don't recall if we've ever bumped into this specific issue. I'll see if i can test it, and i'll release a minor update for Bolt 3.7.x. :-)

[simongroenewolt]

@bobdenotter the content type that resulted in issues for me had a field like this:

routes:
    slug: routes
    name: Routes
    singular_name: Route
    fields:
        // ...
        spaces:
            type: select
            values: (rooms,reflections,connections)/{ref}
            keys: slug
            autocomplete: true
            multiple: true
            sortable: true
            label: Ruimtes