Fix: generator adds new namespace.

[dblock]

Description

Fixes nox -rs generate by properly adding any new namespace in-place of other ones.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.35%. Comparing base (ba715b9) to head (218bc15).
Report is 59 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #813      +/-   ##
==========================================
- Coverage   71.95%   70.35%   -1.61%     
==========================================
  Files          91      113      +22     
  Lines        8001     8877     +876     
==========================================
+ Hits         5757     6245     +488     
- Misses       2244     2632     +388     

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