[zbus-1] zb: Add public exports of dependent crates
This backports a subset of !349 (merged) to zbus-1.x.
Now that zvariant has a new incompatible 3.x major, this allows consumers to use the correct/matching version through the re-export.
The original patchset contained other cleanup commits which unfortunately are heavily tangled in conflicts. However, those aren't strictly required for proper usage.
Many of the structures in zbus use types declared in zbus_names or zvariant, so publicly re-export them in zbus so that users of the library don't need to add extra crates to their Cargo.toml.