DM-48452: Protect subcommand imports via entry points

[timj]

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes
• (if changing dimensions.yaml) make a copy of dimensions.yaml in configs/old_dimensions

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

Project coverage is 89.49%. Comparing base (c03a7ff) to head (7ee97ef).
Report is 4 commits behind head on main.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
python/lsst/daf/butler/cli/butler.py	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1141      +/-   ##
==========================================
- Coverage   89.50%   89.49%   -0.01%     
==========================================
  Files         366      366              
  Lines       49015    49020       +5     
  Branches     5941     5941              
==========================================
  Hits        43872    43872              
- Misses       3728     3733       +5     
  Partials     1415     1415              

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