Inline the use of types

[clarus]

The file https://github.com/paritytech/ink/blob/2eba99ee445a4aee34d0919041388e7e727f7ed8/crates/env/src/api.rs starts with:

use crate::{
    backend::{
        EnvBackend,
        ReturnFlags,
        TypedEnvBackend,
    }

that is translated to:

Module ReturnFlags := crate.backend.ReturnFlags.
Definition ReturnFlags := ReturnFlags.t.

as EnvBackend and TypedEnvBackend are traits and the use of traits is not handled yet.

The goal of this task is to instead inline the use of backend::ReturnFlags so that this type appears fully prefixed where used. This will require introducing an environment of current used types for the translation of top-level items/expressions.

The rationale of this task is to reduce the number of mutual dependencies between sub-modules of a crate, and simplify the re-ordering that we will do later.