Refactor Pipeline into private submodule and improve documentation

[mdekstrand]

This moves Pipeline around a bit:

• Pipeline goes into lenskit.pipeline._impl
• Pipeline warnings and errors are moved to lenskit.diagnostics
• Improve pipeline documentation
• Add support to get_logger to strip out private components of logger names to simplify use of get_logger(__name__) in private implementation modules

[codecov]

Codecov Report

Attention: Patch coverage is 95.85987% with 13 lines in your changes missing coverage. Please review.

Project coverage is 90.74%. Comparing base (91216fd) to head (42a06c8).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
lenskit/lenskit/pipeline/_impl.py	95.54%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #595      +/-   ##
==========================================
+ Coverage   90.72%   90.74%   +0.02%     
==========================================
  Files         106      107       +1     
  Lines        6531     6547      +16     
==========================================
+ Hits         5925     5941      +16     
  Misses        606      606              

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