bridge: When expanding package variables, use package names

Use the real package names when expanding package variables, if
not using checksums.

Closes #1401
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
Rename the old dbus code out of the way

So we can implement new stuff with the usual 'cockpit.dbus' and
CockpitDBusJson names.
doc: Document and design new dbus-json3 protocol payload

There will be additions later, but this is the meat of the
protocol.
bridge: Implement a new dbus-json3 protocol payload

This protocol is much simpler than the others. There will be some
additions to it in the future, but at it's core it's about relaying
messages and signals.

We don't have innate object manager support in the bridge, the
javascript code will respond to events and update an object model
there.
base: Implement a javascript client for new dbus-json3

This implements a simple javascript client for dbus-json3. It has
no proxy support at this time, but that will be added later.
WIP dbus work