Split out _build_grouped_from_imports

[DanielNoord]

More preparation for #2614

By spitting this out (and doing minor improvements to the structure of the code) we make _with_from_imports a bit easier to follow and reuse.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.43%. Comparing base (3d01946) to head (b5c67fa).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2617      +/-   ##
==========================================
- Coverage   99.43%   99.43%   -0.01%     
==========================================
  Files          41       41              
  Lines        3169     3167       -2     
  Branches      686      684       -2     
==========================================
- Hits         3151     3149       -2     
  Misses         11       11              
  Partials        7        7              

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