Moved o_auth/saml/ldap modules under gitlab/auth

What does this MR do?

Move all authentication specific modules like oauth, ldap and saml under the lig/gitlab/auth directory to clean the directory layout.

To have a clean structure it would make more sense to place all these implementations under auth like:

gitlab
|- auth
|  |- oauth
|  |- ldap
|  |- db
|  |- saml

Are there points in the code the reviewer needs to double check?

NA

Why was this MR needed?

This MR resulted as discussion and changes from MR https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8809

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

NA