Previously, the auto-generated impls hard-coded the database's type to
be `&dyn Db`, with `Db` being the local crate's database. Due to
limitations of trait upcasting, this meant that Salsa structs containing
Salsa structs from multiple different crates would not be printed
properly. They would fallback to the plain `std::fmt::Debug` impl, which
just prints an ID -- not very useful.

Now, the impls are generic over the database type. Any database that
implements the local database trait will be accepted.

For more background, see this Zulip conversation [1].

[1]: https://salsa.zulipchat.com/#narrow/stream/145099-general/topic/recursive.20DebugWithDb.3F

Unfortunately, this regresses some unrelated error messages.

The new auto-impl for `DebugWithDb` caused this error message to
regress because it expects the `__salsa_crate_Db` to be in the same
location as the `Jar`. This fixes it by using an absolute path. Note
that using custom `Jar` locations wasn't supported properly before
either.