agent: Resolve checksums in the agent instead of web service

This will make life easier for people embedding Cockpit if they
end up handling the resource channels and resource loading
themselves.
agent: Change the JSON format for listing packages

Allow us to convey information about packages with multiple
names more cleanly and without duplication.
agent: Add support for aliasing packages

This allows a package to claim a well-known package name of a feature
that it provides. It also allows us to mark a certain base package
as the 'latest' etc.

Closes #1316