feat(hugr-py): Allow linking packages and modules from Python

[maximilianruesch]

Adds capabilities to hugr-py to allow linking modules and entire packages together. This is a required feature for Guppy libraries, found in Quantinuum/guppylang#1481, and could be considered the core feature for the same.

hugr-core needed a slight modification to allow reading a module from an envelope (with all the builtin validation around expecting a single module) while also returning the extensions present in the envelope. The functions are exposed via pyo3 bindings to Python and tested there.

[codecov]

Codecov Report

❌ Patch coverage is 62.50000% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.83%. Comparing base (93aac28) to head (8ca8496).

Files with missing lines	Patch %	Lines
hugr-core/src/hugr.rs	0.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2947      +/-   ##
==========================================
- Coverage   83.84%   83.83%   -0.01%     
==========================================
  Files         267      267              
  Lines       52923    52947      +24     
  Branches    46854    46863       +9     
==========================================
+ Hits        44371    44387      +16     
- Misses       6283     6291       +8     
  Partials     2269     2269              

Flag	Coverage Δ
python	88.80% <100.00%> (+0.04%)	⬆️
rust	83.18% <0.00%> (-0.02%)	⬇️

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[maximilianruesch]

Note: I am unsure where to add tests for hugr-core, and which tests to add specifically. Currently, the function is covered by hugr-py tests implicitly.

Also, one could consider whether we should refactor load to call load_with_exts and just ignore the exsts.