DM-47143: Enable entry points for comand-line plugin discovery

[timj]

Requires lsst/daf_butler#1107

Checklist

• added documentation for a new migration script

[codecov]

Codecov Report

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

Project coverage is 59.30%. Comparing base (b93091f) to head (e86f98e).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
...sst/daf/butler_migrate/cli/_get_cli_subcommands.py	0.00%	5 Missing ⚠️
python/lsst/daf/butler_migrate/cli/__init__.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #45      +/-   ##
==========================================
+ Coverage   58.20%   59.30%   +1.09%     
==========================================
  Files          38       40       +2     
  Lines        1407     1413       +6     
  Branches      297      165     -132     
==========================================
+ Hits          819      838      +19     
+ Misses        548      538      -10     
+ Partials       40       37       -3     

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