Since the metadata only provides a date, set the time part to 12:00:00
(UTC).
Also fix up the deprecation warning for the creation of the initial zero
last build timestamp.
This way handlers don't need to do case matching on backend ID strings
anymore.
* Rename `backend` to `backend_id` where we have a backend ID
* Add `get` function and `Backends` enum to the `backend` module
* Add a depend on the `enum_dispatch` crate