Cleanup map importing pipeline

[dabreegster]

A backlog of related things to simplify OSM import.

• From Mateusz: is_road should list known highway values and skip everything else
• Things like bus stop matching are split between convert_osm and map_model/src/make in unclear ways. Consolidate topics when possible.
• Current structure of the pipeline here isn't clear when reading the code. Something better is possible.

[dabreegster]

is_road, get_lane_types, and get_rank are all coupled together in the assumptions they make. It's confusing. I'm considering a consolidated, but possibly much more complicated version that takes Tags and, if the thing should count as a road, returns the lane types in each direction and the rank (used for deciding stop sign priority and for some color schemes). Slight complication is that parking inference happens later in the importer (and it has to stay that way, because you can't decide the closest line to snap to without knowing all the relevant lines).