Let an extension be named apart from its Cargo artifact

[timkpaine]

The Python extension filename was always derived from the Cargo artifact stem, so the cdylib had to be named for the module it becomes. A project whose extension links a core library of the same name then emits two cdylibs called the same thing into target/, which collide on Windows.

Add an optional python-extension-name on an artifact to supply the module stem directly, freeing the Cargo lib name to be unique.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.52%. Comparing base (0f888a0) to head (c739d1f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #73      +/-   ##
==========================================
+ Coverage   86.08%   86.52%   +0.43%     
==========================================
  Files          11       11              
  Lines        1538     1551      +13     
==========================================
+ Hits         1324     1342      +18     
+ Misses        214      209       -5     

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