consider pathway types (including loops) rather than entrances during linking

[abyrd]

pathways.txt proposal:
https://groups.google.com/d/topic/gtfs-changes/JBAT5iFqAlY/discussion

"Pathways of type 1 connect stops and the entrances that give them
access from/to the streets. A stop referenced by a pathway of type 1
can no longer be accessed directly from the streets (i.e., without
using an entrance), unless a circular pathway from the stop to itself
is given to reinstate direct access. Pathways of type 2 describe
pathways for transfers between stops that are separate from the
streets (e.g., a tunnel between two subway platforms). Pathways of
type 3 provide information about transfers on the streets, for use by
transit trip planners instead of, or in addition to, walking
directions from other sources."

We currently just check whether stops are connected to entrances. Instead we should check which pathway types reference them, and whether they have pathways to themselves.

[abyrd]

This should be revisited when the new pathways proposal is finalized.