This enables a fail-early approach, where the remote source loads the initial results.
By storing the `IAsyncEnumerable` it is possible to not perform any additional remote calls, even with the initial probing call.
This makes it possible to detect the absence of sources, so the backup can be aborted if sources are missing.
This fixes the restore issues as all sources will now appear as if they were originally part of the source file system.
The backend implementation has now been simplified so the backends only return names of entries with no path information. Most backends already do this, but the S3 backends did not, so this was fixed as well.
This also clears up the interfaces, so the backend does not know about the `ISourceProviderEntry`. All mapping is done by the `BackendSourceProvider`, making it simpler to support other backends as sources.
Kenneth Skovhede