test(markers): fix the scikit-learn requires_pkg marker

[jdhughes-dev]

requires_pkg() resolves a distribution name and imports the matching module, so
scikit-learn matches neither name on its own — the distribution is scikit-learn and the
module is sklearn. test_save_load_node_mapping_structured was skipped everywhere as a
result, including the nightly optional dependency build, and had never run.

• map the distribution to its module with name_map, as requires_pkg() documents for this case
• the test passes once it runs

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.4%. Comparing base (556c088) to head (e097b31).
⚠️ Report is 209 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2821      +/-   ##
===========================================
+ Coverage     55.5%    73.4%   +17.8%     
===========================================
  Files          644      659      +15     
  Lines       124135   132188    +8053     
===========================================
+ Hits         68947    97044   +28097     
+ Misses       55188    35144   -20044     

see 585 files with indirect coverage changes

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