Simplify some of the OSM->OMT field value mappings using declarative syntax.
This approach is not for all cases, but in many it removes
the need of storing the same field in both the .yaml and .sql files.
TODO: support more complex AND/OR cases
aerodrome_label & mountain_peak queries return `osm_id`, but they
are not declared in the data source. I think we should either remove
the `osm_id` from the query result, or declare the data source.
