node types can be enabled for a channel even if the Search API index doesn't have them

[joachim-n]

When you add a node type to a channel there's no guarantee it will show up, as the Directories search index might not be configured to include it.

The possible options for the included content types field are made by looking at which node types have the directory channel reference field (see #365). It would be less brittle to look at the search API index instead -- although not simple, as we need to allow for Search API datasources other than node entity, such as Date recurrences.