Clean up package auth code

It seems our package managers have some legacy auth code that is either redundant or deprecated somehow.

For example, a few methods in lib/api/helpers/packages/basic_auth_helpers.rb and lib/api/helpers/packages_manager_clients_helpers.rb override or extend existings methods in lib/gitlab/auth/auth_finders.rb and others.

I propose we clean up these methods and integrate it properly in the top-level auth code in auth_finders.