Currently, stage ids are allocated during stage registration and then the parser map is instructed to pass through the id to the `DocumentSourceExtension` constructor. 

However, when a stage comes from a different desugar stage, the parser map isn't used and `DocumentSourceExtension` is instead constructed directly, with no way to obtain the correct id.

To fix this, a static map from stageName to id should be added to DocumentSourceExtension. During stage registration, the id should be added to the map and in the constructor, this id should be looked up (no need to pass id to the constructor).

Additionally, we can get rid of the static id in `DocumentSourceExtension` as it's not necessary (every DocumentSourceExtension doesn't necessarily have the same id).