Partition objects/errors/clients/endpoints into separate top-level modules

[sfackler]

We currently put the generated code for all Conjure types together in the same module. However, once we implement #121, we'll end up generating two traits for each endpoint, and there isn't a clear naming convention I can think of to disambiguate.

We should update the codegen to create separate top-level modules for each Conjure type to keep them separate from each other. For example, using the example API, we would have

use example_api::another::{TestServiceClient, TestService, TestServiceEndpoints, DifferentPackage};

change to

use example_api::{
    clients::another::TestServiceClient,
    endpoints::another::{TestService, TestServiceEndpoints},
    objects::another::DifferentPackage,
};

We could go further and split them into separate example_api_objects, example_api_clients, etc crates like conjure-java does, but that doesn't really seem worth it IMO.