[6.x] Fix nested revealer fields

[duncanmcclean]

This pull request attempts to fix an issue where Revealer fields weren't working in nested fieldtypes like Bard & Replicator.

This was happening because the revealerValues object we're getting from the Container and passing to ShowField looks like this:

{
  replicator.0.revealer: true
}

However, when the field condition validator attempts to get the field's value, it fails, because it's trying to find a value using the revealer's field handle, not its "full dotted path".

This PR fixes it by only passing the "relative" revealer values to ShowField and flattening the keys to just the field handles.

Fixes #12099.