Investigate yaml_source_map confusion

[handrews]

The yaml_source_map package gets confused by the github API description file in YAML format. However, its counterpart json_source_map has no problem with the description file once it is converted to JSON.

Currently, the code issues a warning and just gives up on the source map if it errors out, but it should probably either be fixed (with a PR to the project) or re-implemented if that seems impractical.