Fix requirements using locals of a top-level modular scenario

[dfremont]

Description

Fixes #392, wherein an internal error resulted when a require statement in a modular scenario referred to one of the scenario's local variables. The bug actually only happened when the modular scenario was at the top level, not invoked in the compose block of another scenario, but I've added tests for both cases.

Issue Link

Fixes #392.

Checklist

• I have tested the changes locally via pytest and/or other means
• I have added or updated relevant documentation
• I have autoformatted the code with black and isort
• I have added test cases (if applicable)

Additional Notes

This PR only fixes the specific issue in #392, but I think same kind of issue will happen when other kinds of mutable objects are used inside requirements, e.g. dicts. I'm not sure how to fix the problem in all cases, so I will leave it for a future PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.73%. Comparing base (3c5102e) to head (b897b05).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #455      +/-   ##
==========================================
+ Coverage   89.71%   89.73%   +0.02%     
==========================================
  Files          48       48              
  Lines       13194    13211      +17     
==========================================
+ Hits        11837    11855      +18     
+ Misses       1357     1356       -1     

Files with missing lines	Coverage Δ
src/scenic/core/dynamics/scenarios.py	94.79% <100.00%> (+0.19%)	⬆️
src/scenic/core/requirements.py	96.02% <100.00%> (+0.03%)	⬆️

... and 3 files with indirect coverage changes

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