feat: add an option to exclude packages from freezing

[ajkerrigan]

It's useful to be able to freeze most dependencies but leave specific packages with deliberately looser/intact version constraints.

Testing from the command level seems necessary (with or without the motivation of coverage failures), but it's definitely more awkward. Tried to cobble together the minimum useful test based on looking at resources like:

• Cleo's testing commands docs
• How poetry-plugin-export builds on top of Cleo's base testing support

That command-level test feels like it could be made clearer/simpler/better, but at this point I'm not sure how :thinking:

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.08%. Comparing base (9f1ef28) to head (a594ffe).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #16 +/- ## ========================================== + Coverage 90.59% 97.08% +6.49% ========================================== Files 1 1 Lines 202 206 +4 ========================================== + Hits 183 200 +17 + Misses 19 6 -13 ```

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