Fix namespacing in Katello::Authorization::Product

These changes fix an error related to products that occurs when a user
issues the hammer product command in the v2 API.

The error only occurs the first time after hitting the web server
running under foreman. See also:

  - https://github.com/theforeman/hammer-cli-katello/pull/11
  - https://github.com/theforeman/hammer-cli-katello/pull/12

The error stated the index action of the ProductsController couldn't
find readable? method in the Katello::Authorization::Provider module.
This error morphed to show a different undefined method as I added
namespace prefixes in Katello::Authorization::Product.

**See also**: (You'll need these changes to reproduce the error)

https://github.com/theforeman/hammer-cli-katello/pull/11
https://github.com/theforeman/hammer-cli-katello/pull/12