We previously had the module registry protocol documented only as an
undefined subset of the full API of the official registry implementation.
However, the vast majority of endpoints documented in the official API
docs are not needed for a headless third-party module registry that only
intends to make modules available to Terraform CLI.
To make this clearer to potential third-party implementors, and also for
consistency with how the provider registry protocol is now documented,
here we create a new page to describe the subset required for all
registries, and then explain in the docs for the offical API that
potential third-party implementors should refer to the new page instead.
The longer page describing the full API of the official implementations
remains for those who wish to write clients for that API, because it is
part of the API surface area for Terraform Cloud and Terraform Enterprise.
I also took this opportunity to address the fact that module addresses
don't really contain "provider names" at all, but rather than the fourth
field in the address is _conventionally_ an official provider name but
can really be any string that serves to differentiate multiple
implementations of the same abstraction. The new docs therefore refer to
this field as "system" rather than "provider".
Martin Atkins