This is based on my understanding of how it currently works, and
this comment by @moranegg: https://forge.softwareheritage.org/D637#inline-3321
I introduced the following changes:
* introduced the concept of "gatherer" as a metadata provider, as a catch-all
for everything that does not fit in the other types, eg. if we decide on
implementing this: https://forge.softwareheritage.org/T833#31999
We can discard it afterward if we don't need it.
* Dropped the concept of 'tool'. As far as I understand, they are an
intermediary between the provider and the SWH archive. IMO they only
complicate stuff if we plan on extracting metadata from providers without
* Changed the described endpoints to drop the concept of "provider id"
(you know I don't like extrinsic identifiers by now :) )
* Changed the described endpoints to give consistent names to the parameters.