[TODO: remove in v15] Duplicate HTTP Auth Basic and HTTP Auth Digest tools have to be merged #1688

While working on #1680 I realized that there's functional duplication in tools we provide by default.
There's auth_basic, auth_digest and also basic_auth, digest_auth.
They also have duplicate tests spread across 3-5 test modules, which has to be handled as well.

#913 (0704e6c) and as mentioned in #914:

Note that with this new auth_digest and the auth_basic in ticket #913, the files lib/auth.py and lib/httpauth.py can be completely deprecated.

• I'm submitting a ...

• bug report
• feature request
• question about the decisions made in the repository
• WAT

• Do you want to request a feature or report a bug?
  Deprecated tool removal.

• What is the current behavior?
  There's ambiguity making end-users confused about which tool to use.

• If the current behavior is a bug, please provide the steps to reproduce and if possible a screenshots and logs of the problem.
  See src

• What is the expected behavior?
  Just one well-documented and well-tested implementation for each of Basic/Digest HTTP Auth methods.

• What is the motivation / use case for changing the behavior?
  There should be one-- and preferably only one --obvious way to do it. (Zen of Python)

• Please tell us about your environment:

• Cheroot version: —
• CherryPy version: 3.2.0‒13.1.0+
• Python version: —
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• Other information (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, e.g. stackoverflow, gitter, etc.)
  —