Backend trait refactors

- Refactor the confusing/bodgy consts backend macro to use a single
  `BackendInfo` struct
- Combine `supports_as_dependency()` and the `switches_dependency` into a single `Option` type as that is a better representation as it is an optional backend feature.
- Add more types like `Packages`, `Sections` for uniformity.
- Added a crate-private prelude and switches all imports to use a
  prelude glob import (excluding functions because it's nice to only be
  importing types)
- Refactored Backend section name in logging by implementing Display for
  `AnyBackend` that just prints the `section`

crates/pacdef/src/grouping/mod.rs

@@ -9,11 +9,6 @@ all groups using [`Group::load`], which in turn will get all packages from all
 sections.
 */
 
-mod group;
-mod package;
-mod section;
-
-pub use group::Group;
-pub use group::Groups;
-pub use package::Package;
-pub use section::Section;
+pub mod group;
+pub mod package;
+pub mod section;