Cleanup: move the generated code to isolate the "use" statement and cleanup all the artificial _name introduced

[gbin]

today the main proc macro generated imports within the user module.
That is not optimal as the user might want to also import those modules.

the solution is to move all the generated code in a submodule and export back only the symbol needed by the user:

mod generated_name {
use cu29::something ...
generated_struct
}
pub use generated_struct;