index: plot_keys: relparts -> parts

[efiop]

_plot_sources is messed up and needs a rewrite hard, but this will do for now for studio purposes.
Rewrite and tests later on in a followup.

Related https://github.com/iterative/studio/pull/6541

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.27% ⚠️

Comparison is base (f4c0946) 90.40% compared to head (d17beb7) 90.14%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9820      +/-   ##
==========================================
- Coverage   90.40%   90.14%   -0.27%     
==========================================
  Files         484      484              
  Lines       36887    36887              
  Branches     5326     5326              
==========================================
- Hits        33349    33250      -99     
- Misses       2928     3002      +74     
- Partials      610      635      +25     

Files Changed	Coverage Δ
dvc/repo/index.py	85.01% <0.00%> (ø)

... and 26 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.