Codegen: Standardize and implement `--packages` handling for all backend

[bladyjoker]

Currently, Haskell and Purescript backends handle foreign LB references without considering the package they are in. This is something the Codegen can't decide and has to be provided by the user (Nix wrappers basically collect and provide this information).

However, because Haskell and Purescript don't care about packages when handling imports, this problem didn't arise until we started implementing Rust and Typescript backends.

Regardless, I think it should be done the same way everywhere, so let's implement the same --packages and pkgMap :: Map LB.ModuleName (BackendPackageName backend) and properly resolve this.

I'd even say let's use PackageImports in Haskell so it's abundantly clear where every module is sourced from.