It started as simple as that I would like to look up all possible providers. Which ones are allowed? However, where is the full struct definition?
There isn't a place that I am aware of where the documentation is hosted outside of the GitHub repo here.
I just dig through those and the actual implementation files if needs be to figure out what is available to me if I'm looking for/at new features.
As of WWDC 2019 there is now documentation on Apple's developer website for the package manager. It is here at the time of updating this.