Local repositories no longer convert manifest files into a JSON file
(that matched the format of the remote repositories JSON data).
Instead the data is loaded directly from the manifest files.
This is necessary to support read-only "System" repositories.
Use a class to generalize loading from both data sources to simplify the
implementation which was previously inlined.
This fixes an error where stale data from the manifests was
sometimes used because the generated JSON was considered up to date.
a24add4a66