This commit reorganises the code for both the registry/API and the
Orchestrator/Registry.
The main difference with the previous version is how stuff is exposed.
Now we only expose a Registry interface to the outside (previously named
Orchestrator), which has several implementations: null is the default,
and is returned if HCP is not enabled.
The other implementations being HCL/JSON, both private to the hcp
sub-package.
The api (previously `registry') is the set of functionality that
abstracts and calls the HCP API.
It was meant to be merged with the `hcp' package, but because of a
dependency loop with the datasources, both are separated for now.
