[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.

[robdekort]

Would love to see this one merged if possible.

[andjsch]

@jasonvarga Any chance this makes it to the next release?

[jasonvarga]

This reverts the edge case bug (I've opened in #13622) but we can revisit that when time permits. I'd rather have this out there.