use field map in LocalgovDirectoriesEntryTypes::getReferenceableEntities()

[joachim-n]

LocalgovDirectoriesEntryTypes::getReferenceableEntities() returns those node types which have the 'localgov_directory_channels' field on them. It's doing this by looping over all node_type entities, and then getting ALL the fields on each node type.

This is fairly expensive. It's much easier to use EntityFieldManagerInterface::getFieldMap(). The list of node types that have the localgov_directory_channels field is then given by $field_map['node']['localgov_directory_channels']['bundles'].

(Though there is other brittleness here too -- filing an issue for that in a moment.)