Dropping legacy databake interface

[robertbastian]

All interesting changes are in the first commit

[sffc]

Seems fine but I thought we preferred #[path] over include!

[robertbastian]

We prefer

#[path = "foo.rs"]
mod foo;

over

mod foo {
  include!("foo.rs");
}

but we don't need the nested mods here.

[robertbastian]

#2856