[REQUEST] Implement schedule agencies ID verification (GTFS rule)

[maximearmstrong]

Is your feature request related to a problem? Please describe. A schedule should not have IDless agencies, except if there's only a single agency defined. Agencies ID must be valid. This is a GTFS rule implemented in Google Python validator as ValidateIdlessAgency and ValidateRouteAgencyId.

Describe the solution you'd like Actual Google GTFS validator behaviour : Verifies if an agency is IDless. An agency can be IDless only if a single agency is defined. Verifies if route agencies ID are valid (in agencies set). https://github.com/google/transitfeed/blob/master/transitfeed/schedule.py line 1281

Describe alternatives you've considered

Additional context Line 171 and 172 in Error support priorities https://docs.google.com/spreadsheets/d/1vqe6wq7ctqk1EhYkgtZ0_TbcQ91vccfs2daSjn20BLE/edit#gid=0

[lionel-nj]

Implemented in https://github.com/MobilityData/gtfs-validator/pull/269

[ghost]

@lionel-nj can that issue be closed? Let me know if that's NOT the case

[lionel-nj]

@fabrice-v As for parity with the python validator, this issue can be closed. However verification of foreign key agency_id remains to do for GTFS attributions.txt and fare_attributes.txt. Keeping this issue opened as a reminder.

[lionel-nj]

Closing this issue, will backport implementation to check validity of fare_attributes.agency_id and attributions.agency_id in a future PR.

@fabrice-v