Guard against import machinery masking out attributes

[stefanv]

Thank you to @matthew-brett for uncovering and analyzing this issue.

This deals with the case where a function my_func is defined in my_func.py. In some cases where the my_func submodule is imported (e.g. from pkg.my_func import ..., import pkg.my_func, or import * accessing a sibling attribute in the same file) the import machinery updates the package __dict__ with the my_func (the submodule) overriding access to my_func (the function).

This wraps the package __setattr__ to intercept those (and only those) problematic assignments.

[codecov]

Codecov Report

❌ Patch coverage is 76.92308% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.54%. Comparing base (af0adfc) to head (e788bb1).

Files with missing lines	Patch %	Lines
src/lazy_loader/__init__.py	76.92%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              main     #171      +/-   ##
===========================================
- Coverage   100.00%   97.54%   -2.46%     
===========================================
  Files            1        1              
  Lines          109      122      +13     
  Branches        19       23       +4     
===========================================
+ Hits           109      119      +10     
- Misses           0        1       +1     
- Partials         0        2       +2     

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