Remove undocumented modules containing one item

[SabrinaJewson]

Description

This removes the modules auth_code, auth_code_pkce, client_creds, clients::base and clients::oauth from the public API surface.

Motivation and Context

These modules are unnecessary because they only export a single item which has the same name as the module itself anyway. Documentation is easier to navigate if they’re not present.

I left out mutex because I have other plans for that module.

Dependencies

None.

Type of change

• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How has this been tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce.

Please also list any relevant details for your test configuration

• Test A: cargo test --features env-file

Is this change properly documented?

• Will add a changelog entry once I have a PR number.

[marioortizmanero]

Awesome! That should clean up the docs a bit.

Thanks for the PR!