index/features: Simplify split_features() fn

[Turbo87]

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[codecov]

Codecov Report

Attention: Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.69%. Comparing base (2eec9bb) to head (32c93d9).
Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/crates_io_index/features.rs	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9650      +/-   ##
==========================================
- Coverage   88.69%   88.69%   -0.01%     
==========================================
  Files         288      288              
  Lines       29368    29366       -2     
==========================================
- Hits        26048    26046       -2     
  Misses       3320     3320              

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